[code coverage] Add code_coverage_in_gerrit md doc

This CL add a md doc for code coverage in Gerrit.

Change-Id: I889f545468eb1da433f1dc857a4e50c385d54e14
Reviewed-on: https://chromium-review.googlesource.com/c/1438123
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627099}

docs/code_coverage_in_gerrit.md

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+# Code Coverage in Gerrit
+
+**Have you ever wanted to know if your CL has enough code coverage?** You can
+see uncovered lines in Gerrit, which can help you figure out if your CL or the
+CL you're reviewing needs more/better tests.
+
+To generate code coverage data for your CL, **manually** trigger the
+experimental coverage CQ builder: *linux-coverage-rel*:
+
+![choose_tryjobs] ![linux_coverage_rel]
+
+Once the build finishes and code coverage data are processed successfully, look
+at the right column of the side by side diff view to see which lines are **not**
+covered:
+
+![uncovered_lines]
+
+## Contacts
+
+### Reporting problems
+For any breakage report and feature requests, please [file a bug].
+
+### Mailing list
+For questions and general discussions, please join [code-coverage group].
+
+## FAQ
+### Why is the coverage CQ builder needs to be triggered manually?
+
+We're still evaluating the stability of the code coverage pipeline, and once
+it's proven to be stable, the experimental builder will be merged into one of
+the existing CQ builders, and at that time, the feature will be available by
+default.
+
+### Why is coverage not shown even though the try job finished successfully?
+
+There are several possible reasons:
+* All the added/modified lines are covered.
+* A particular source file/test may not be available on a particular project or
+platform. As of now, only `chromium/src` project and `Linux` platform is
+supported.
+* There is a bug in the pipeline. Please [file a bug] to report the breakage.
+
+### How does it work?
+
+Please refer to [code_coverage.md] for how code coverage works in Chromium in
+general, and specifically, for per-CL coverage in Gerrit, the
+[clang_code_coverage_wrapper] is used to compile and instrument ONLY the source
+files that are affected by the CL and a [chromium-coverage Gerrit plugin] is
+used to annotate uncovered lines in Gerrit.
+
+
+[choose_tryjobs]: images/code_coverage_choose_tryjobs.png
+[linux_coverage_rel]: images/code_coverage_linux_coverage_rel.png
+[uncovered_lines]: images/code_coverage_uncovered_lines.png
+[file a bug]: https://bugs.chromium.org/p/chromium/issues/entry?components=Tools%3ECodeCoverage
+[code-coverage group]: https://groups.google.com/a/chromium.org/forum/#!forum/code-coverage
+[code_coverage.md]: code_coverage.md
+[clang_code_coverage_wrapper]: clang_code_coverage_wrapper.md
+[chromium-coverage Gerrit plugin]: https://chromium.googlesource.com/infra/gerrit-plugins/code-coverage/