Add plugin documentation page

docs/source/index.rst

@@ -31,6 +31,7 @@ Welcome to rez's documentation!
    managing_packages
    caching
    pip
+   plugins
 
 .. toctree::
    :maxdepth: 2

docs/source/plugins.rst

@@ -0,0 +1,66 @@
+=======
+Plugins
+=======
+
+Rez is designed around the concept of plugins. Plugins can be used to extend rez's functionalities without modifying any of rez's source code.
+
+Plugins are currently bundled in the main rez repo, but will be split out
+to their own repos in the future.
+
+The built-in plugins are located at :gh-rez:`src/rezplugins`.
+
+Existing plugins
+================
+
+- :gh-rez:`src/rezplugins/build_process`
+- :gh-rez:`src/rezplugins/build_system`
+- :gh-rez:`src/rezplugins/command`
+- :gh-rez:`src/rezplugins/build_process/package_repository`
+- :gh-rez:`src/rezplugins/build_process/release_hook`
+- :gh-rez:`src/rezplugins/build_process/release_vcs`
+- :gh-rez:`src/rezplugins/build_process/shell`
+
+Developing your own plugin
+==========================
+
+Rez plugins require a specific folder structure as follows:
+
+.. code-block:: text
+
+    /plugin_name
+        /__init__.py (adds plugin path to rez)
+        /rezconfig.py (defines configuration settings for your plugin)
+        /plugin_file1.py (your plugin file)
+        /plugin_file2.py (your plugin file)
+        etc.
+
+To make your plugin available to rez, you can install them directly under
+``src/rezplugins`` (that's called a namespace package) or you can add
+the path to :envvar:`REZ_PLUGIN_PATH`.
+
+Plugin types
+------------
+
+There are two different plugin types in Rez.
+
+- Plugin
+- Extension
+
+Extensions differ from standard plugins in that they add a new command to rez's
+CLI.
+
+Required file contents
+----------------------
+``__init__.py``
+
+.. code-block:: python
+
+    from rez.plugin_managers import extend_path
+    __path__ = extend_path(__path__, __name__)
+
+``your_plugin_file.py``
+
+.. code-block:: python
+
+    def register_plugin():
+        return YourPluginClass