[libfuzzer] updating documentation with mac support.

testing/libfuzzer/reference.md

 # libFuzzer Integration Reference
 
+## Supported Platforms and Configurations
+
+### Linux
+
+Linux is fully supported by libFuzzer and ClusterFuzz with following sanitizer 
+configurations:
+
+| GN Argument | Description |
+|--------------|----|
+| is_asan=true | enables [Address Sanitizer] to catch problems like buffer overruns. |
+| is_msan=true | enables [Memory Sanitizer] to catch problems like uninitialed reads. |
+| is_ubsan_security=true | enables [Undefined Behavior Sanitizer] to catch<sup>\[[1](#Notes)\]</sup> undefined behavior like integer overflow. |
+
+Configuration example:
+
+```bash
+# With address sanitizer
+gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false' --check
+```
+
+### Mac
+
+Mac is experimentally supported by libFuzzer with `is_asan` configuration. Mac
+support is not provided by ClusterFuzz.
+
+Configuration example:
+
+```bash
+gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false mac_deployment_target="10.7"' --check
+```
+
+
 ## fuzzer_test GN Template
 
 Use `fuzzer_test` to define libFuzzer targets:
 
 ```
 fuzzer_test("my_fuzzer") {
   ...
 }
 ```
 
@@ skipping 34 matching lines @@
 fuzzer_test("my_fuzzer") {
   ...
   libfuzzer_options = [
     "max_len=2048",
     "use_traces=1",
   ]
 }
 ```
 
 [libFuzzer Usage]: http://llvm.org/docs/LibFuzzer.html#usage
+[Address Sanitizer]: http://clang.llvm.org/docs/AddressSanitizer.html
+[Memory Sanitizer]: http://clang.llvm.org/docs/MemorySanitizer.html
+[Undefined Behavior Sanitizer]: http://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
 
-