Add doc/note/fuzzing.md

doc/note/fuzzing.md

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+# Fuzzing
+
+Wuffs has been fuzzed (on [OSS-Fuzz](https://google.github.io/oss-fuzz/)) since
+[February
+2018](https://github.com/google/oss-fuzz/commit/27f374480b1e7c780d161b7a6727c23cc03419c5).
+
+As of November 2019, this has not yet found any memory-safety bugs in Wuffs.
+Compile-time safety checks seem to work.
+
+However, it has found a few correctness bugs, based on generating random input.
+Wuffs' fuzzer programs will [intentionally
+segfault](https://github.com/google/wuffs/blob/7ec252876541ec203659949450fafddc148b606e/fuzz/c/fuzzlib/fuzzlib.c#L61)
+if the Wuffs library returns an "internal" error message.
+
+For example, Wuffs [won't let you write `x -=
+1`](/doc/note/interval-arithmetic.md#i--i--1-doesnt-compile) unless you can
+prove that it won't underflow. The programmer might 'know' that `x` is always
+positive at some point in their program, but to satisfy Wuffs' [bounds
+checker](/doc/note/bounds-checking.md), they have to explicitly write something
+like:
+
+```
+if x > 0 {
+    x -= 1
+} else {
+    // Unreachable: x is always positive because blah blah blah.
+    return "#internal error: etc"
+}
+```
+
+Hitting the else branch means that there's a bug: what the programmer 'knew'
+was incorrect. These aren't security bugs per se: decoding an image would
+produce the wrong pixels, or abort early, instead of leading to RCE (Remote
+Code Execution). But fuzzing Wuffs has still been useful.