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/ModHelper/ModHelperPlugin.php

@@ -32,7 +32,7 @@ class ModHelperPlugin extends Plugin
         $versions[] = array('name' => 'ModHelper',
                             'version' => GNUSOCIAL_VERSION,
                             'author' => 'Brion Vibber',
-                            'homepage' => 'http://status.net/wiki/Plugin:ModHelper',
+                            'homepage' => 'https://git.gnu.io/gnu/gnu-social/tree/master/plugins/ModHelper',
                             'rawdescription' =>
                             // TRANS: Plugin description.
                             _m('Lets users who have been manually marked as "modhelper"s silence accounts.'));