Add fuzzing!

Reviewed-by: Emilia Käsper <emilia@openssl.org>

fuzz/README.md

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+# I Can Haz Fuzz?
+
+Or, how to fuzz OpenSSL with libfuzzer.
+
+Starting from a vanilla+OpenSSH server Ubuntu install.
+
+Use Chrome's handy recent build of clang. Older versions may also work.
+
+    $ sudo apt-get install git
+    $ mkdir git-work
+    $ git clone https://chromium.googlesource.com/chromium/src/tools/clang
+    $ clang/scripts/update.py
+
+You may want to git pull and re-run the update from time to time.
+
+Update your path:
+
+    $ PATH=~/third_party/llvm-build/Release+Asserts/bin/:$PATH
+
+Get and build libFuzzer (there is a git mirror at
+https://github.com/llvm-mirror/llvm/tree/master/lib/Fuzzer if you prefer):
+
+    $ cd
+    $ sudo apt-get install subversion
+    $ mkdir svn-work
+    $ cd svn-work
+    $ svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
+    $ cd Fuzzer
+    $ clang++ -c -g -O2 -std=c++11 *.cpp
+    $ ar r libFuzzer.a *.o
+    $ ranlib libFuzzer.a
+
+Configure for fuzzing:
+
+    $ CC=clang ./config enable-fuzz enable-asan enable-ubsan no-shared
+    $ sudo apt-get install make
+    $ LDCMD=clang++ make -j
+    $ fuzz/helper.py <fuzzer> <arguments>
+
+Where `<fuzzer>` is one of the executables in `fuzz/`. Most fuzzers do not
+need any command line arguments, but, for example, `asn1` needs the name of a
+data type.
+
+If you get a crash, you should find a corresponding input file in
+`fuzz/corpora/<fuzzer>-crash/`. You can reproduce the crash with
+
+    $ fuzz/<fuzzer> <crashfile>