fuzz/README.md: add a brief README

fuzz/README.md

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+# Tectonic Fuzzing Support
+
+This is *beta-level* support for [fuzzing] the Tectonic engine using
+[cargo-fuzz]. At the moment, the fuzzing does not work very well because it
+exposes memory leaks in the Tectonic engine; this is desirable in and of
+itself, but the exposed leaks have proven to be tricky to fix. (Patches most
+welcome, of course! The limiting factor here is definitely developer
+bandwidth.)
+
+See the script `./run-fuzzer.sh` for an example of how to, well, run the
+fuzzer. Note that this script sets a [rustup directory override] to build
+Tectonic using the nightly compilers, since nightly is currently required by
+the fuzzer.
+
+To tell the fuzzer to ignore memory leaks, run a command like the following
+from the toplevel directory of the Tectonic repository:
+
+```sh
+cargo +nightly fuzz run compile fuzz/corpus fuzz/seeds -- -detect_leaks=0
+```
+
+[fuzzing]: https://en.wikipedia.org/wiki/Fuzzing
+[cargo-fuzz]: https://github.com/rust-fuzz/cargo-fuzz
+[rustup directory override]: https://github.com/rust-lang/rustup.rs#directory-overrides