Create docs for fuzzing

fuzz/README.md

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+# Fuzzing
+
+Fuzz tests generate a ton of random parameter arguments to the program and then validate that none cause it to crash.
+
+## How does it work?
+
+Typically, Travis CI will run `travis-fuzz.sh` on one of the environments the automated tests are configured for.
+This is the most time-consuming component of the continuous integration workflow, so it is recommended that you detect
+issues locally, and Travis merely acts as a sanity check. Fuzzing is further only effective with
+a lot of CPU time, indicating that if crash scenarios are discovered on Travis with its low
+runtime constraints, the crash is caused relatively easily.
+
+## How do I run fuzz tests locally?
+
+You typically won't need to run the entire combination of different fuzzing tools. For local execution, `honggfuzz`
+should be more than sufficient. 
+
+### Setup
+
+To install `honggfuzz`, simply run
+
+```shell
+cargo update
+cargo install --force honggfuzz
+```
+
+### Execution
+
+To run the Hongg fuzzer, do
+
+```shell
+export CPU_COUNT=1 # replace as needed
+export HFUZZ_BUILD_ARGS="--features honggfuzz_fuzz"
+export HFUZZ_RUN_ARGS="-n $CPU_COUNT --exit_upon_crash"
+
+export TARGET="msg_ping_target" # replace with the target to be fuzzed
+cargo hfuzz run $TARGET 
+```
+
+To see a list of available fuzzing targets, run:
+
+```shell
+ls ./src/bin/
+```
+
+## A fuzz test failed on Travis, what do I do?
+
+You're trying to create a PR, but need to find the underlying cause of that pesky fuzz failure blocking the merge?
+
+Worry not, for this is easily traced.
+
+If your Travis output log looks like this:
+
+```
+Size:639 (i,b,hw,ed,ip,cmp): 0/0/0/0/0/1, Tot:0/0/0/2036/5/28604
+Seen a crash. Terminating all fuzzing threads
+
+… # a lot of lines in between
+
+<0x0000555555565559> [func:UNKNOWN file: line:0 module:/home/travis/build/rust-bitcoin/rust-lightning/fuzz/hfuzz_target/x86_64-unknown-linux-gnu/release/full_stack_target]
+<0x0000000000000000> [func:UNKNOWN file: line:0 module:UNKNOWN]
+=====================================================================
+2d3136383734090101010101010101010101010101010101010101010101
+010101010100040101010101010101010101010103010101010100010101
+0069d07c319a4961
+The command "if [ "$(rustup show | grep default | grep stable)" != "" ]; then cd fuzz && cargo test --verbose && ./travis-fuzz.sh; fi" exited with 1.
+```
+
+Note that the penultimate stack trace line ends in `release/full_stack_target]`. That indicates that
+the failing target was `full_stack`. To reproduce the error locally, simply copy the hex, 
+and run the following from the `fuzz` directory:
+
+```shell
+export TARGET="full_stack" # adjust for your output
+export HEX="2d3136383734090101010101010101010101010101010101010101010101\
+010101010100040101010101010101010101010103010101010100010101\
+0069d07c319a4961" # adjust for your output
+
+mkdir -p ./test_cases/$TARGET
+echo $HEX | xxd -r -p > ./test_cases/$TARGET/any_filename_works
+
+export RUST_BACKTRACE=1
+cargo test
+```
+
+This will reproduce the failing fuzz input and yield a usable stack trace.