doc: add a document about our fuzz testing integration

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

doc/FUZZING.md

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+# Fuzz testing
+
+C-lightning currently supports coverage-guided fuzz testing using [LLVM's libfuzzer](https://www.llvm.org/docs/LibFuzzer.html)
+when built with `clang`.
+
+The goal of fuzzing is to generate mutated -and often unexpected- inputs (`seed`s) to pass
+to (parts of) a program (`target`) in order to make sure the codepaths used:
+- do not crash
+- are valid (if combined with sanitizers)
+The generated seeds can be stored and form a `corpus`, which we try to optimise (don't
+store two seeds that lead to the same codepath).
+
+For more info about fuzzing see [here](https://github.com/google/fuzzing/tree/master/docs),
+and for more about `libfuzzer` in particular see [here](https://www.llvm.org/docs/LibFuzzer.html).
+
+
+## Build the fuzz targets
+
+In order to build the C-lightning binaries with code coverage you will need a recent
+[clang](http://clang.llvm.org/). The more recent the compiler version the better.
+
+Then you'll need to enable support at configuration time. You likely want to enable
+a few sanitizers for bug detections as well as experimental features for an extended
+coverage (not required though).
+
+```
+DEVELOPER=1 EXPERIMENTAL_FEATURES=1 ASAN=1 UBSAN=1 VALGRIND=0 FUZZING=1 CC=clang ./configure && make
+```
+
+The targets will be built in `tests/fuzz/` as `fuzz-` binaries.
+
+
+## Run one or more target(s)
+
+You can run each target independently. Pass `-help=1` to see available options, for
+example:
+```
+./tests/fuzz/fuzz-addr -help=1
+```
+
+Otherwise, you can use the Python runner to either run the targets against a given seed
+corpus:
+```
+./tests/fuzz/run.py fuzz_corpus -j2
+```
+Or extend this corpus:
+```
+./tests/fuzz/run.py fuzz_corpus -j2 --generate --runs 12345
+```
+
+The latter will run all targets two by two `12345` times.
+
+If you want to contribute new seeds, be sure to merge your corpus with the main one:
+```
+./tests/fuzz/run.py my_locally_extended_fuzz_corpus -j2 --generate --runs 12345
+./tests/fuzz/run.py main_fuzz_corpus --merge_dir my_locally_extended_fuzz_corpus
+```
+
+
+## Write new fuzzing targets
+
+In order to write a new target:
+ - include the `libfuzz.h` header
+ - fill two functions: `init()` for static stuff and `run()` which will be called
+     repeatedly with mutated data.
+ - read about [what makes a good fuzz target](https://github.com/google/fuzzing/blob/master/docs/good-fuzz-target.md).
+
+A simple example is [`fuzz-addr`](tests/fuzz/fuzz-addr.c). It setups the chainparams and
+context (wally, tmpctx, ..) in `init()` then bruteforces the bech32 encoder in `run()`.