import plugins from core and restructure repo

Readme.rst

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+Pelican Plugins
+###############
+
+Beginning with version 3.0, Pelican supports plugins. Plugins are a way to add
+features to Pelican without having to directly modify the Pelican core. Starting
+with 3.2, all plugins (including the ones previously in the core) are 
+moved here, so this is the central place for all plugins. 
+
+How to use plugins
+==================
+
+Easiest way to install and use these plugins is cloning this repo::
+
+    git clone https://github.com/getpelican/pelican-plugins
+
+and activating the ones you want in your settings file::
+
+    PLUGIN_PATH = 'path/to/pelican-plugins'
+    PLUGINS = ['assets', 'sitemap', 'gravatar']
+
+``PLUGIN_PATH`` can be a path relative to your settings file or an absolute path.
+
+Alternatively, if plugins are in an importable path, you can omit ``PLUGIN_PATH``
+and list them::
+
+    PLUGINS = ['assets', 'sitemap', 'gravatar']
+
+or you can ``import`` the plugin directly and give that::
+
+    import my_plugin
+    PLUGINS = [my_plugin, 'assets']
+
+Please refer to the ``Readme`` file in a plugin's folder for detailed information about 
+that plugin.
+
+Contributing a plugin
+=====================
+
+Please refer to the `Contributing`_ file.
+
+.. _Contributing: Contributing.rst