Add an 'About' section in the documentation
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

akilman/appd-hackathon-2016,pgroudas/gerrit-keep-approvals,pgroudas/gerrit-keep-approvals,akilman/appd-hackathon-2016,GerritCodeReview/plugins_cookbook-plugin,GerritCodeReview/plugins_cookbook-plugin,GerritCodeReview/plugins_cookbook-plugin,akilman/appd-hackathon-2016