Add instructions for building the fuzzing corpus

- Inform devs how to build an organized and useful corpora based on the
results of fuzz testing.
- Modify .gitignore to allow corpus/ tracking. Note: files in corpus/
  should not be blindly committed but selectively added based on
  usefulness
- Add emptyfile to zkas-decoder corpus because it caused a panic

fuzz/.gitignore

@@ -1,5 +1,4 @@
 target
-corpus
 artifacts
 coverage
 Cargo.lock

fuzz/README.md

@@ -0,0 +1,38 @@
+# DarkFi Fuzzing
+
+This directory contains our fuzz tests. It is a WIP and likley to be
+re-organized as we expand the complexity of the tests
+
+## Building the corpora
+
+### Motivation
+If you discover a crash while fuzzing, add it to the relevant
+subdirectory in `corpus/` and give it a meaningful name.
+
+Files in the corpora will be used as default inputs in subsequent
+runs in the fuzzer. The fuzzer will then "mutate" or modify these
+inputs using various algorithms to create new yet similar inputs.
+This is a way to get more value from fuzzing as we'll be able to
+test using inputs similar to ones that have been problematic in the
+past and therefore more likely to find bugs.
+
+Another benefit is that we will be able to detect regressions
+in the codebase by simply running our known corpora against the fuzzer
+and making sure the code doesn't crash.
+
+Finally, the corpora make for good building blocks for unit tests 
+as they represent known error cases that the code has had at some point.
+
+### Example
+e.g. scenario: while testing ZkBinary's decode() function, you find
+that an empty input causes a panic.
+
+* Identify your fuzz target (`cargo fuzz list` or whatever you used
+for `cargo fuzz run TARGET`
+* `ls artifacts/TARGET/crash-*`
+* `cat` the crash file and check that it matches the error message from
+the fuzzer
+* Choose a `NAME` for the crash file, e.g. `corpus_emptyfile`
+* `cp artifacts/TARGET/CRASH-FILE corpus/TARGET/NAME`
+
+Then add the new `corpus/TARGET/NAME` file to git.