Added microscope plugin structure

docs/source/plugins.rst

@@ -44,9 +44,33 @@ For example, the plugins section of your microscoperc.yaml file may look like:
 
 Microscope plugin structure
 ---------------------------
-.. note::
+As described earlier, a plugin must subclass :py:class:`openflexure_microscope.plugins.MicroscopePlugin`. Each plugin in described by a single class, containing any number of methods. The methods defined within your plugin class will be attached to the microscope. By subclassing :py:class:`openflexure_microscope.plugins.MicroscopePlugin`, your methods automatically get access to the microscope object your plugin is attached to, through `self.microscope`. This is an instance of :py:class:`openflexure_microscope.Microscope`, and gives unrestricted access to the attached camera and stage hardware.
 
-   To be written
+For example, a simple plugin file named `myplugin.py`, may look like:
+
+.. code-block:: python
+
+    from openflexure_microscope.plugins import MicroscopePlugin
+
+    class MyPlugin(MicroscopePlugin):
+
+        def identify(self):
+            """
+            Demonstrate access to Microscope.camera, and Microscope.stage
+            """
+
+            response = "My parent camera is {}, and my parent stage is {}.".format(self.microscope.camera, 
+                                                                                   self.microscope.stage)
+            return response
+
+        def hello_world(self):
+            """
+            Demonstrate passive method
+            """
+
+            return "Hello world!"
+
+When this plugin is loaded and attached to a microscope object named `microscope`, the plugin methods will be available at `microscope.plugin.myplugin.identify()` and `microscope.plugin.myplugin.hello_world()`.
 
 Adding web API routes
 ---------------------