Add README.md with an overview of the plugin and links to docs

Change-Id: I814a58035e3c52d072454aa9e72f42975bb6550c

README.md

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+Gerrit Gravatar plugin
+======================
+
+This plugin populates Gerrit users' avatar images from the popular Gravatar platform using the MD5
+code of the user's preferred email.
+
+How to build
+------------
+
+Clone or link this plugin to the plugins directory of Gerrit's source tree,
+and then run bazel build on the plugin's directory.
+
+Example:
+
+```
+git clone --recursive https://gerrit.googlesource.com/gerrit
+cd plugins
+git clone "https://gerrit.googlesource.com/modules/avatars-gravatar"
+cd .. && bazelisk build plugins/avatars-gravatar
+```
+
+The output plugin jar is created in `bazel-bin/plugins/avatars-gravatar/avatars-gravatar.jar`
+
+How to install
+--------------
+
+Copy the `avatars-gravatar.jar` into the `${GERRIT_SITE}/plugins/` and start
+Gerrit Code Review.
+
+Configuration
+-------------
+
+The plugin configuration is read from $GERRIT_SITE/etc/gerrit.config, see
+the [config.md](src/main/resources/Documentation/config.md) documentation for
+more details on the individual settings.