[libfuzzer] store custom options in .GN build target instead of a separate file.

testing/libfuzzer/efficient_fuzzer.md

Index: testing/libfuzzer/efficient_fuzzer.md
diff --git a/testing/libfuzzer/efficient_fuzzer.md b/testing/libfuzzer/efficient_fuzzer.md
index 5eb0b6ed2bd203fdae933653ef015cada526dcdb..7ea08d3c121ffb2a4cef9430eedde261df5975f2 100644
--- a/testing/libfuzzer/efficient_fuzzer.md
+++ b/testing/libfuzzer/efficient_fuzzer.md
@@ -89,6 +89,7 @@ magic numbers etc. The easiest way to diagnose this problem is to generate a
 * change the code (e.g. disable crc checks while fuzzing)
 * prepare [corpus seed](#Corpus-Seed)
 * prepare [fuzzer dictionary](#Fuzzer-Dictionary)
+* specify [custom options](#Custom-Options)
 
 ## Coverage
 
@@ -167,6 +168,28 @@ Make sure to submit dictionary file to git. The dictionary will be used
 automatically by ClusterFuzz once it picks up new fuzzer version (once a day).
 
 
+### Custom Options
+
+It is possible to specify [libFuzzer parameters](http://llvm.org/docs/LibFuzzer.html#usage)
+for any fuzzer being run at ClusterFuzz. Custom options will overwrite default
+values provided by ClusterFuzz.
+
+Just list all parameters in `libfuzzer_options` variable of build target:
+
+```
+fuzzer_test("my_protocol_fuzzer") {
+  ...
+  libfuzzer_options = [
+    "max_len=2048",
+    "use_traces=1",
+  ]
+}
+```
+
+Please note that `dict` parameter should be provided [separately](#Fuzzer-Dictionary).
+Other options may be passed through `libfuzzer_options` property.
+
+
 [ClusterFuzz status]: ./clusterfuzz.md#Status-Links
 [upload corpus to ClusterFuzz]: ./clusterfuzz.md#Upload-Corpus
 [AFL]: http://lcamtuf.coredump.cx/afl/