Add documentation for creating new fuzz test targets.

Author: arik-so

fuzz/README.md

@@ -84,4 +84,36 @@ export RUSTFLAGS="--cfg=fuzzing"
 cargo test
 ```
 
+Note that if the fuzz test failed locally, moving the offending run's trace 
+to the `test_cases` folder should also do the trick, simply replace the `echo $HEX |` line above
+with (the trace file name is of course a bit longer than in the example):
+
+```shell
+mv hfuzz_workspace/fuzz_target/SIGABRT.PC.7ffff7e21ce1.STACK.[…].fuzz ./test_cases/$TARGET/
+```
+
 This will reproduce the failing fuzz input and yield a usable stack trace.
+
+
+## How do I add a new fuzz test?
+
+1. The easiest approach is to take one of the files in `fuzz/src/`, such as e. g.
+`process_network_graph.rs`, and duplicate it, renaming the new file to something more 
+suitable. For the sake of example, let's call the new fuzz target we're creating 
+`my_fuzzy_experiment`.
+
+2. In the newly created file `fuzz/src/my_fuzzy_experiment.rs`, run a string substitution
+of `process_network_graph` to `my_fuzzy_experiment`, such that the three methods in the
+file are `do_test`, `my_fuzzy_experiment_test`, and `my_fuzzy_experiment_run`.
+
+3. In `fuzz/src/bin/gen_target.sh`, add a line reading `GEN_TEST my_fuzzy_experiment` to the 
+first group of `GEN_TEST` lines (starting in line 9).
+
+4. If your test relies on a new local crate, add that crate to `fuzz/Cargo.toml`.
+
+5. In `fuzz/src/lib.rs`, add the line `pub mod my_fuzzy_experiment`. Additionally, if 
+you added a new crate dependency, add the `extern crate […]` import line.
+
+6. Run `fuzz/src/bin/gen_target.sh`.
+
+7. There is no step seven: happy fuzzing!