fuzzing: Document how to use honggfuzz to fuzz libnbd.

fuzzing/README

@@ -1,5 +1,6 @@
 To report security bugs, see ‘SECURITY’ in the top source directory.
 
+
 Fuzzing libnbd using the American Fuzzy Lop (afl) fuzzer
 ========================================================
 
@@ -58,6 +59,27 @@ command below. You can run this even while afl-fuzz is running.
 This will create an HTML test coverage report in
 ../libnbd-afl/fuzzing/sync_dir/cov/web/
 
+
+Fuzzing libnbd using honggfuzz
+==============================
+
+Recompile libnbd with honggfuzz instrumentation:
+
+ ./configure \
+ CC=/path/to/hfuzz-clang CXX=/path/to/hfuzz-clang++ \
+ --disable-shared \
+ --disable-golang --disable-ocaml --disable-python
+ make clean
+ make
+
+Run honggfuzz using test cases:
+
+ honggfuzz -i fuzzing/testcase_dir -z -- \
+ ./fuzzing/libnbd-fuzz-wrapper ___FILE___
+
+(Note 3 underscore characters on each side.)
+
+
 Fuzzing libnbd using Clang + libFuzzer
 ======================================