Update Readme.md (#6402)

* Update Readme.md

- Reorganize readme for code structure

* Update Readme.md

Readme.md

@@ -3,7 +3,7 @@ Open Asset Import Library (assimp)
 
 Open Asset Import Library is a library that loads various 3D file formats into a shared, in-memory format. It supports more than __40 file formats__ for import and a growing selection of file formats for export.
 
-### Current project status ###
+### Current project status
 ![C/C++ CI](https://github.com/assimp/assimp/workflows/C/C++%20CI/badge.svg)
 [![Codacy Badge](https://app.codacy.com/project/badge/Grade/9973693b7bdd4543b07084d5d9cf4745)](https://www.codacy.com/gh/assimp/assimp/dashboard?utm_source=github.com&utm_medium=referral&utm_content=assimp/assimp&utm_campaign=Badge_Grade)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=assimp_assimp&metric=alert_status)](https://sonarcloud.io/summary/new_code?id=assimp_assimp)
@@ -16,32 +16,32 @@ Open Asset Import Library is a library that loads various 3D file formats into a
 APIs are provided for C and C++. Various bindings exist to other languages (C#, Java, Python, Delphi, D). Assimp also runs on Android and iOS.
 Additionally, assimp features various __mesh post-processing tools__: normals and tangent space generation, triangulation, vertex cache locality optimization, removal of degenerate primitives and duplicate vertices, sorting by primitive type, merging of redundant materials and many more.
 
-## Project activity ##
+## Project activity
 ![Alt](https://repobeats.axiom.co/api/embed/997f84e5f9fcf772da1e687f3a4f3a8afdbf4cf0.svg "Repobeats analytics image")
 
-### Documentation ###
+### Documentation
 Read [our latest documentation](https://the-asset-importer-lib-documentation.readthedocs.io/en/latest/).
 
-### Pre-built binaries ###
+### Pre-built binaries
 Download binaries from [our Itch Projectspace](https://kimkulling.itch.io/the-asset-importer-lib).
 
-### Test data ###
+### Test data
 Clone [our model database for testing purposes](https://github.com/assimp/assimp-mdb).
 
-### Communities ###
+### Communities
 - Ask questions at [the Assimp Discussion Board](https://github.com/assimp/assimp/discussions).
 - Find us on [discord](https://discord.gg/kKazXMXDy2)
 - Ask [the Assimp community on Reddit](https://www.reddit.com/r/Assimp/).
 - Ask on [StackOverflow with the assimp-tag](http://stackoverflow.com/questions/tagged/assimp?sort=newest).
 - Nothing has worked? File a question or an issue report at [The Assimp-Issue Tracker](https://github.com/assimp/assimp/issues)
 
-#### Supported file formats ####
+#### Supported file formats
 See [the complete list of supported formats](https://github.com/assimp/assimp/blob/master/doc/Fileformats.md).
 
-### Building ###
+### Building
 Start by reading [our build instructions](https://github.com/assimp/assimp/blob/master/Build.md). We are available in vcpkg, and our build system is CMake; if you used CMake before there is a good chance you know what to do.
 
-### Ports ###
+### Ports
 * [Android](port/AndroidJNI/README.md)
 * [Python](port/PyAssimp/README.md)
 * [.NET](https://github.com/Saalvage/AssimpNetter)
@@ -54,35 +54,39 @@ Start by reading [our build instructions](https://github.com/assimp/assimp/blob/
 * [HAXE-Port](https://github.com/longde123/assimp-haxe) The Assimp-HAXE-port.
 * [Rust](https://github.com/jkvargas/russimp)
 
-### Other tools ###
+### Other tools
 [Qt5-ModelViewer](https://github.com/sharjith/ModelViewer-Qt5) is a powerful viewer based on Qt5 and Assimp's import and export abilities.<br>
 [Assimp-Viewer](https://github.com/assimp/assimp_view) is an experimental implementation for an Asset-Viewer based on ImGUI and Assimp (experimental).
 
 #### Repository structure ####
 Open Asset Import Library is implemented in C++. The directory structure looks like this:
 
-	/code		Source code
-	/contrib	Third-party libraries
-	/doc		Documentation (Doxygen source and pre-compiled docs)
-	/fuzz       Contains the test code for the Google Fuzzer project
-	/include	Public header C and C++ header files
-	/scripts 	Scripts are used to generate the loading code for some formats
-	/port		Ports to other languages and scripts to maintain those.
-	/test		Unit- and regression tests, test suite of models
-	/tools		Tools (old assimp viewer, command line `assimp`)
-	/samples	A small number of samples to illustrate possible use cases for Assimp
+```txt
+code				Source code
+├── AssetLib/		The asset-importer and exporter lib
+├── CApi/			C-API files
+├── Common/			Common code used from all modules
+├── Geometry/		Geometry utilities
+├── Material/		Common materials
+├── Pbrt/			Physical based materials
+├── PostProcessing/	Post-processing steps
+├── res				Resouce files
+contrib				Third-party libraries
+doc					Documentation (Doxygen source and pre-compiled docs)
+fuzz       			Contains the test code for the Google Fuzzer project
+include				Public header C and C++ header files
+scripts 			Scripts are used to generate the loading code for some formats
+port				Ports to other languages and scripts to maintain those.
+test				Unit- and regression tests, test suite of models
+├── headercheck		Implements headerchecks
+├── models-nonbsd	Non-BSP licensed headers
+├── models			BSP-licensed models
+├── unit			Unit tests
+tools				Tools (old assimp viewer, command line `assimp`)
+samples				Small number of samples to illustrate possible use cases for Assimp
+```
+### Contributing
 
-The source code is organized in the following way:
-
-	code/Common			        The base implementation for importers and the infrastructure
-	code/CApi                   Special implementations which are only used for the C-API
-	code/Geometry               A collection of geometry tools
-	code/Material               The material system
-	code/PBR                    An exporter for physical-based models
-	code/PostProcessing		    The post-processing steps
-	code/AssetLib/<FormatName>	Implementation for import and export of the format
-
-### Contributing ###
 We would greatly appreciate for you to contribute to assimp. The easiest way to get involved is to submit
 a pull request with your changes against the main repository's `master` branch.