Add initial documentation for ClusterFuzz.

docs/clusterfuzz.md

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+# ClusterFuzz
+
+ClusterFuzz is the infrastructure behind OSS-Fuzz, and started as fuzzing
+infrastructure for Chrome.
+
+## Web interface
+
+The [web interface](https://clusterfuzz-external.appspot.com/v2) also provides a
+web interface to view statistics about your fuzzer targets, as well as current
+crashes.
+
+Note that access is restricted to library developers that we auto CC on new bug
+reports.
+
+## Fuzzer stats
+
+You can view statistics about your fuzzers (e.g. speed, coverage information) on
+our fuzzer statistics dashboard.
+
+![stats]
+(freetype_stats.png)
+
+## Coverage reports
+
+You can also visualise the coverage for your fuzzer by going to our coverage
+report links, where we highlight the parts of source code that are being
+reached.
+
+![coverage_1]
+(freetype_coverage_1.png)
+
+![coverage_2]
+(freetype_coverage_2.png)
+
+## Testcase reports
+
+ClusterFuzz will automatically de-duplicate and file reproducible crashes into
+our [bug tracker](https://bugs.chromium.org/p/monorail). We provide a crash
+report page that gives you the stack trace, a link to the crashing testcase, and
+regression ranges where the bug was most likely introduced.
+
+![report]
+(pcre2_testcase.png)
+