Merge branch 'readme-plugins' into 'nightly'

Readme plugins

* Adds several plugin READMEs
  They are pretty basic, but it's a start.

* Changes status.net/wiki URLs to git.gnu.io
  The status.net wiki is dead.

See merge request !103

diff --cc plugins/FacebookBridge/FacebookBridgePlugin.php
Simple merge

diff --cc plugins/ModHelper/ModHelperPlugin.php
Simple merge

diff --cc plugins/OStatus/OStatusPlugin.php
Simple merge

diff --cc plugins/OpenID/OpenIDPlugin.php
Simple merge

diff --cc plugins/RegisterThrottle/RegisterThrottlePlugin.php
Simple merge

diff --cc plugins/StrictTransportSecurity/StrictTransportSecurityPlugin.php
Simple merge

diff --cc plugins/TwitterBridge/TwitterBridgePlugin.php
Simple merge