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: I04670af6dbec0dffe6b21434dd3f8815d0eee9dd

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
new file mode 100644
index 0000000..f6d763f
--- /dev/null
+++ b/src/main/resources/Documentation/about.md
@@ -0,0 +1,2 @@
+Sample plugin to demonstrate features of Gerrit's plugin API.
+