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

testing/libfuzzer/efficient_fuzzer.md

 # Efficient Fuzzer
 
 This document describes ways to determine your fuzzer efficiency and ways 
 to improve it.
 
 ## Overview
 
 Being a coverage-driven fuzzer, libFuzzer considers a certain input *interesting*
 if it results in new coverage. The set of all interesting inputs is called 
 *corpus*. 
@@ skipping 101 matching lines @@
 
 ```
 ./out/libfuzzer/my_fuzzer ~/tmp/my_fuzzer_corpus
 ```
 
 The directory can initially be empty. The fuzzer would store all the interesting
 items it finds in the directory. You can help the fuzzer by "seeding" the corpus:
 simply copy interesting inputs for your function to the corpus directory before
 running. This works especially well for strictly defined file formats or data
 transmission protocols.
+
 * For file-parsing functionality just use some valid files from your test suite.
+
 * For protocol processing targets put raw streams from test suite into separate
 files.
 
-After discovering new and interesting items, [upload corpus to ClusterFuzz].
+
+ClusterFuzz uses seed corpus stored in Chromium repository. You need to add
+`seed_corpus` attribute to fuzzer target:
+
+```
+fuzzer_test("my_protocol_fuzzer") {
+  ...
+  seed_corpus = "src/fuzz/testcases"
+  ...
+}
+```
+
+If you don't want to store seed corpus in Chromium repository, you can upload
+corpus to Google Cloud Storage bucket used by ClusterFuzz:
+
+
+1) go to [Corpus GCS Bucket]
+
+2) open directory named `%YOUR_FUZZER_NAME%_static`
+
+3) upload corpus files into the directory
+
+
+Alternative way is to use `gsutil` tool:
+```bash
+gsutil -m rsync <corpus_dir_on_disk> gs://clusterfuzz-corpus/libfuzzer/%YOUR_FUZZER_NAME%_static
+```
+
 
 ### Fuzzer Dictionary
 
 It is very useful to provide fuzzer a set of common words/values that you expect
 to find in the input. This greatly improves efficiency of finding new units and
 works especially well while fuzzing file format decoders.
 
 To add a dictionary, first create a dictionary file.
 Dictionary syntax is similar to that used by [AFL] for its -x option:
 
@@ skipping 46 matching lines @@
     "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/
+[ClusterFuzz status]: clusterfuzz.md#Status-Links
+[Corpus GCS Bucket]: https://goto.google.com/libfuzzer-clusterfuzz-corpus