libfuzzer: add documentation recommending how to silence error logging

testing/libfuzzer/getting_started.md

Index: testing/libfuzzer/getting_started.md
diff --git a/testing/libfuzzer/getting_started.md b/testing/libfuzzer/getting_started.md
index cfe1ab25aa46f2a811c1c5f31da03e170d12dbd2..7f7c2335e5ff5ab6a242964f77c0975638de1ca2 100644
--- a/testing/libfuzzer/getting_started.md
+++ b/testing/libfuzzer/getting_started.md
@@ -104,6 +104,23 @@ You can specify custom `max_len` value to be used by ClusterFuzz. For more
 information check out [Maximum Testcase Length] section of the [Efficient Fuzzer
 Guide].
 
+## Remove error message logging

[inferno, 2016/08/30 15:57:39]

s/Remove/Disable noisy

[robert.bradford, 2016/08/30 16:16:51]

Done.

+
+If the code that you are a fuzzing generates error messages when encountering
+incorrect or invalid data then you need to silence those errors in the fuzzer.
+The best way to do that is to override the environment used for logging in your

[mmoroz, 2016/08/30 15:54:00]

I think it's worth to clarify "If the target uses Chromium logging interface,
...". Because some targets (usually third party libraries) have other logging
functions and this trick won't work there. 

[robert.bradford, 2016/08/30 16:16:51]

Done.

+fuzzer:
+
+```cpp
+struct Environment {
+  Environment() {
+    logging::SetMinLogLevel(logging::LOG_FATAL);
+  }
+};
+
+Environment* env = new Environment();
+```
+
 ## Submitting Fuzzer to ClusterFuzz
 
 ClusterFuzz builds and executes all `fuzzer_test` targets in the source tree.