fuzz: Add README

fuzz/README.rst

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+Fuzzer
+======
+
+This directory contains fuzzer target mainly written to integrate
+nghttp2 into `oss-fuzz <https://github.com/google/oss-fuzz>`_.
+
+fuzz_target.cc contains an entry point of fuzzer.  corpus directory
+contains initial data for fuzzer.
+
+The file name of initial data under corpus is the lower-cased hex
+string of SHA-256 hash of its own content.
+
+corpus/h2spec contains input data which was recorded when we ran
+`h2spec <https://github.com/summerwind/h2spec>`_ against nghttpd.
+
+corpus/nghttp contains input data which was recorded when we ran
+nghttp against nghttpd with some varying command line options of
+nghttp.
+
+
+To build fuzz_target.cc, make sure that libnghttp2 is built with
+following compiler/linker flags:
+
+.. code-block:: text
+
+    CPPFLAGS="-fsanitize-coverage=edge -fsanitize=addres"
+    LDFLAGS="-fsanitize-coverage=edge -fsanitize=addres"
+
+Then, fuzz_target.cc can be built using the following command:
+
+.. code-block:: text
+
+    $ clang++ -fsanitize-coverage=edge -fsanitize=address -I../lib/includes -std=c++11 fuzz_target.cc ../lib/.libs/libnghttp2.a  /usr/lib/llvm-3.9/lib/libFuzzer.a -o nghttp2_fuzzer