docs: plugin; renaming

docs/index.rst

@@ -28,6 +28,7 @@ PDF is the best format for storing and exchanging scanned documents.  Unfortunat
    docker
    advanced
    api
+   plugins
    batch
    security
    errors

docs/plugins.rst

@@ -0,0 +1,22 @@
+Plugins
+=======
+
+You can use plugins to customize the behavior of OCRmyPDF at certain points of interest.
+
+Currently, it is possible to:
+- override the decision for whether or not to perform OCR on a particular file
+- modify the image is about to be sent for OCR
+
+How plugins are imported
+------------------------
+
+Plugins are imported on demand, by the OCRmyPDF worker process that needs to use them.
+As such, plugins cannot share state with each other, and will be imported many times,
+once for each worker process.
+
+Plugins currently cannot override the same hook.
+
+How plugins are invoked
+-----------------------
+
+Plugins may be called from the command line: