docs/internals/PORTING.md: new document

The basic approach I use when porting libcurl to another OS when the
existing configure or cmake build setups are not suitable.

Closes #16860

docs/Makefile.am

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  internals/MQTT.md                              \
  internals/MULTI-EV.md                          \
  internals/NEW-PROTOCOL.md                      \
+ internals/PORTING.md                           \
  internals/README.md                            \
  internals/SPLAY.md                             \
  internals/STRPARSE.md                          \

docs/internals/PORTING.md

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+<!--
+Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+
+SPDX-License-Identifier: curl
+-->
+
+# porting libcurl
+
+The basic approach I use when porting libcurl to another OS when the existing
+configure or cmake build setups are not suitable.
+
+## Build
+
+Write a build script/Makefile that builds *all* C files under lib/. If
+possible, use the `lib/Makefile.inc` that lists all files in Makefile
+variables.
+
+In the Makefile, make sure you define what OS you build for: `-D[OPERATING
+SYSTEM]`, or similar. Perhaps the compiler in use already define a standard
+one? Then you might not need to define your own.
+
+## Add the new OS
+
+In the `lib/curl_config.h` header file, in the section for when `HAVE_CONFIG_H`
+is *not* defined (starting at around line 150), add a new conditional include
+in this style:
+
+~~~c
+#ifdef [OPERATING SYSTEM]
+#  include "config-operatingsystem.h"
+#endif
+~~~
+
+Create `lib/config-operatingsystem.h`. You might want to start with copying a
+another config-* file and then start trimming according to what your
+environment supports.
+
+## Build it
+
+When you run into compiler warnings or errors, the
+`lib/config-operatingsystem.h` file should be where you should focus your work
+and edits.
+
+A recommended approach is to define a lot of the `CURL_DISABLE_*` defines (see
+the [CURL-DISABLE](../CURL-DISABLE.md) document) initially to help narrow down
+the initial work as that can save you from having to give attention to areas of
+the code that you do not care for in your port.