Added plugins FAQ to the docs

docs/plugins.rst

@@ -1,4 +1,48 @@
 Developing new plugins
 ----------------------
 
-See the examples at: `<http://github.com/gsi-upm/senpy-plugins-community>`_.
+Plugins Interface
+=================
+
+The basic methods in a plugin are:
+
+* __init__
+* activate: used to load memory-hungry resources
+* deactivate: used to free up resources
+
+Plugins are loaded asynchronously, so don't worry if the activate method takes too long. The plugin will be marked as activated once it is finished executing the method.
+
+F.A.Q.
+======
+If I'm using a classifier, where should I train the classifier?
+???????????????????????????????????????????????????????????????
+
+Training a classifier can be time time consuming. To avoid running the training unnecessarily, you can use ShelfMixin to store the classifier. For instance:
+
+.. code:: python
+
+          from senpy.plugins import ShelfMixin, SenpyPlugin
+
+          class MyPlugin(ShelfMixin, SenpyPlugin):
+              def train(self):
+                  ''' Code to train the classifier
+                  '''
+                  # Here goes the code
+                  # ...
+                  return classifier
+
+              def activate(self):
+                  if 'classifier' not in self.sh:
+                      classifier = self.train()
+                      self.sh['classifier'] = classifier
+                  self.classifier = self.sh['classifier']
+              
+              def deactivate(self):
+                  self.close()
+
+You can speficy a 'shelf_file' in your .senpy file. By default the ShelfMixin creates a file based on the plugin name and stores it in that plugin's folder.
+
+Where can I find more code examples?
+????????????????????????????????????
+
+See: `<http://github.com/gsi-upm/senpy-plugins-community>`_.