Add README

Add a README.md file in the repository root with the same content as
the "About" page.

Gitiles will render the README by default when the project is viewed
on the googlesource site [1] and this will make the configuration
documentation and list of supported hooks more easily accessible for
users who have not installed the plugin yet in their Gerrit site.

[1] https://gerrit.googlesource.com/plugins/hooks

Change-Id: Ib2ecf818cbabaca8bfddf58e12cce99842a4f038

README.md

diff --git a/README.md b/README.md
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index 0000000..70f6774
--- /dev/null
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+# Hooks plugin
+
+Gerrit does not support the [standard server-side git hooks][1] in the
+repositories it manages.
+
+This plugin adds support for custom hooks that can be run instead. Refer
+to the [configuration documentation][2] and [list of supported hooks][3]
+for details.
+
+[1]: https://git-scm.com/book/gr/v2/Customizing-Git-Git-Hooks
+[2]: src/main/resources/Documentation/config.md
+[3]: src/main/resources/Documentation/hooks.md