Add an 'About' section in the documentation

Add a new file `about.md` with a brief description of the plugin
that will be included at the top of the plugin's documentation
index.

Change-Id: Ie56e96e387950061a49fb6e56d1001c715d190e2
diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
new file mode 100644
index 0000000..ded934b
--- /dev/null
+++ b/src/main/resources/Documentation/about.md
@@ -0,0 +1,3 @@
+Stores review information for Gerrit changes in the refs/notes/review
+branch.
+