Add a brief overview of fuzzing/Bitcoin Core fuzzing

- Google's repo
- Our report
- John's advice on fuzz-friendly development

Author: agroce

doc/fuzzing.md

@@ -19,6 +19,10 @@ $ FUZZ=process_message src/test/fuzz/fuzz
 There is also a runner script to execute all fuzz targets. Refer to
 `./test/fuzz/test_runner.py --help` for more details.
 
+## Overview of Bitcoin Core fuzzing
+
+[Google](https://github.com/google/fuzzing/) has a good overview of fuzzing in general, with contributions from key architects of some of the most-used fuzzers. [This paper](https://agroce.github.io/bitcoin_report.pdf) includes an external overview of the status of Bitcoin Core fuzzing, as of summer 2021.  [John Regehr](https://blog.regehr.org/archives/1687) provides good advice on writing code that assists fuzzers in finding bugs, which is useful for developers to keep in mind.
+
 ## Fuzzing harnesses and output
 
 [`process_message`](https://github.com/bitcoin/bitcoin/blob/master/src/test/fuzz/process_message.cpp) is a fuzzing harness for the [`ProcessMessage(...)` function (`net_processing`)](https://github.com/bitcoin/bitcoin/blob/master/src/net_processing.cpp). The available fuzzing harnesses are found in [`src/test/fuzz/`](https://github.com/bitcoin/bitcoin/tree/master/src/test/fuzz).