Add READMEs

fuzz/README.md

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+# Fuzzing
+
+This folder contains a Cargo package for a fuzz target that can be used for fuzzing with [american fuzzy lop](https://github.com/google/AFL). Fuzzing has found many rare bugs and unhandled edge cases that cause crashes and is invaluable for improving the reliability of hyperbuild.
+
+## Inputs
+
+Initial inputs can be found in the [in](./in) folder.
+
+## Building
+
+```bash
+cargo afl build
+```
+
+## Running
+
+```bash
+cargo afl fuzz -i in -o out target/debug/hyperbuild-fuzz-target
+```
+
+## Results
+
+Inputs that cause a crash are found in the `out/crashes` folder.