fuzz: add fuzzing README

also remove `set -e` from `fuzz-util.sh` because the README suggests
that users sometimes source this into their interactive shell.

fuzz/README.md

@@ -0,0 +1,68 @@
+# Fuzzing
+
+`bitcoin` and `bitcoin_hashes` have fuzzing harnesses setup for use with
+honggfuzz.
+
+To run the fuzz-tests as in CI -- briefly fuzzing every target -- simply
+run
+
+    ./fuzz.sh
+
+in this directory.
+
+To build honggfuzz, you must have libunwind on your system, as well as
+libopcodes and libbfd from binutils **2.38** on your system. The most
+recently-released binutils 2.39 has changed their API in a breaking way.
+
+On Nix, you can obtain these libraries by running
+
+    nix-shell -p libopcodes_2_38 -p libunwind
+
+and then run fuzz.sh as above.
+
+# Long-term fuzzing
+
+To see the full list of targets, the most straightforward way is to run
+
+    source ./fuzz-util.sh
+    listTargetNames
+
+To run each of them for an hour, run
+
+    ./cycle.sh
+
+To run a single fuzztest indefinitely, run
+
+    HFUZZ_BUILD_ARGS='--features honggfuzz_fuzz' cargo hfuzz run <target>
+
+This script uses the `chrt` utility to try to reduce the priority of the
+jobs. If you would like to run for longer, the most straightforward way
+is to edit `cycle.sh` before starting. To run the fuzz-tests in parallel,
+you will need to implement a custom harness.
+
+# Adding fuzz tests
+
+All fuzz tests can be found in the `fuzz_target/` directory. Adding a new
+one is as simple as copying an existing one and editing the `do_test`
+function to do what you want.
+
+If your test clearly belongs to a specific crate, please put it in that
+crate's directory. Otherwise you can put it directly in `fuzz_target/`.
+
+If you need to add dependencies, edit the file `generate-files.sh` to add
+it to the generated `Cargo.toml`.
+
+Once you've added a fuzztest, regenerate the `Cargo.toml` and CI job by
+running
+
+    ./generate-files.sh
+
+Then to test your fuzztest, run
+
+    ./fuzz.sh <target>
+
+If it is working, you will see a rapid stream of data for many seconds
+(you can hit Ctrl+C to stop it early). If not, you should quickly see
+an error.
+
+

fuzz/fuzz-util.sh

@@ -1,13 +1,11 @@
 #!/usr/bin/env bash
 
-set -e
-
 REPO_DIR=$(git rev-parse --show-toplevel)
 
 listTargetFiles() {
-  pushd "$REPO_DIR/fuzz" > /dev/null
+  pushd "$REPO_DIR/fuzz" > /dev/null || exit 1
   find fuzz_targets/ -type f -name "*.rs"
-  popd > /dev/null
+  popd > /dev/null || exit 1
 }
 
 targetFileToName() {