[libfuzzer] Update documentation on UBSan options and Upload Corpus section.

testing/libfuzzer/clusterfuzz.md

Index: testing/libfuzzer/clusterfuzz.md
diff --git a/testing/libfuzzer/clusterfuzz.md b/testing/libfuzzer/clusterfuzz.md
index 513b45eaf2d2f6b169695af6f9f9cdb7bca34234..58c1b91216367a665f85df257536ddd8b47ca5e3 100644
--- a/testing/libfuzzer/clusterfuzz.md
+++ b/testing/libfuzzer/clusterfuzz.md
@@ -35,6 +35,20 @@ metrics as well as provides links to crashes and coverage reports. The informati
 is collected every 30 minutes.
 
 
+## Corpus
+ClusterFuzz uses two corpus types with libFuzzer:
+
+* **Seed** (or **static**) corpus: files manually uploaded by developers.
+ClusterFuzz uses these files for fuzzing but doesn't delete/overwrite them.
+
+* **General** (or **working**) corpus: files generated by fuzzers themselves.
+These corpus files are frequently modified during fuzzing sessions and can be
+deleted during corpus minimization.
+
+A fuzzer has two input corpus directories, seed and general, but its output
+goes into general corpus directory. Seed corpus is read-only.
+
+
 [Buildbot]: https://goto.google.com/libfuzzer-clusterfuzz-buildbot
 [fuzzer_test.gni]: https://code.google.com/p/chromium/codesearch#chromium/src/testing/libfuzzer/fuzzer_test.gni
 [chromium_libfuzzer.py]: https://code.google.com/p/chromium/codesearch#chromium/build/scripts/slave/recipes/chromium_libfuzzer.py