Add "About" page

Change-Id: I33a4f4402647303f9952161072918423eba53072
diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
new file mode 100644
index 0000000..acd2778
--- /dev/null
+++ b/src/main/resources/Documentation/about.md
@@ -0,0 +1 @@
+This plugin reports Gerrit metrics to Graphite.