[libfuzzer] support multiple seed_corpus directories.

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 7c3be85dc794874c004ddaecf1dda9c126973d42..b14c7c0574385b81065eeb7637e7136c30f03934 100644
--- a/testing/libfuzzer/efficient_fuzzer.md
+++ b/testing/libfuzzer/efficient_fuzzer.md
@@ -58,6 +58,19 @@ fuzzer_test("my_protocol_fuzzer") {
 }
 ```
 
+You may specify multiple seed corpus directories via `seed_corpuses` attribute:
+
+```
+fuzzer_test("my_protocol_fuzzer") {
+  ...
+  seed_corpuses = [ "src/fuzz/testcases", "src/unittest/data" ]
+  ...
+}
+```
+
+All files found in the directories and their subdirectories will be archived
+into `%YOUR_FUZZER_NAME%_seed_corpus.zip` output archive.
+
 If you don't want to store seed corpus in Chromium repository, you can upload
 corpus to Google Cloud Storage bucket used by ClusterFuzz: