fuzzing: Add readme.

fuzzing/README.md

+# Directory structure
+
+- `fuzzers/`: Sources for the fuzzing harnesses.
+- `corpora/`: Contains the fuzzing corpora and assorted tools. The corpora are
+  filed under a directory with the same name as the fuzzing harness. Each of
+  those directories also contains the following:
+
+  - `in/`: Contains the actual corpus test cases.
+  - `tools/`: Any tools useful for that particular harness. A good example
+    would be a binary which generates seed test cases.