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

plugins/FacebookBridge/FacebookBridgePlugin.php

@@ -594,7 +594,7 @@ ENDOFSCRIPT;
             'name' => 'Facebook Bridge',
             'version' => GNUSOCIAL_VERSION,
             'author' => 'Craig Andrews, Zach Copley',
-            'homepage' => 'http://status.net/wiki/Plugin:FacebookBridge',
+            'homepage' => 'https://git.gnu.io/gnu/gnu-social/tree/master/plugins/FacebookBridge',
             'rawdescription' =>
              // TRANS: Plugin description.
             _m('A plugin for integrating StatusNet with Facebook.')