Add basic docs on plugins

PLUGINS.md

@@ -0,0 +1,31 @@
+# Plugins
+
+Various use cases of JavaRosa can be extended with plugins.
+
+## Parse
+
+Intercept parts of the process and attach data to the created `FormDef` through its extras.
+
+### API
+- `XFormParser#addProcessor`
+- `FormDef#getExtras`
+
+The default `XFormParser` can be overridden by creating an implementation of `IXFormParserFactory` (`IXFormParserFactory.Wrapper` if you want to chain multiple plugins) and calling `XFormUtils.setXFormParserFactory` with it.
+
+## Finalization
+
+Inspect the `FormEntryModel` after finalization (or "post processing") and attach data via its extras.
+
+### API
+- `FormEntryController#addPostProcessor`
+- `FormEntryModel#getExtras`
+
+## Instance
+
+Inspect external instances (their ID and parsed XML) after parsing or provide custom parsers for specific instances or file types.
+
+### API
+- `ExternalInstanceParser#addFileInstanceParser`
+- `ExternalInstanceparsser#addProcessor`
+
+The default `ExternalInstanceParser` can be overridden by creating an implementation of `ExternalInstanceParserFactory` and calling `XFormUtils.setExternalInstanceParserFactory` with it.

README.md

@@ -30,6 +30,10 @@ Snapshots versions are also published for each commit to `master` and are availa
 
 JavaRosa works on Android API level 21+ (with desugaring enabled) and Java 8+.
 
+## Extending
+
+JavaRosa is intended be extended to support extra features through a "plugin" approach documented [here](PLUGINS.md) .
+
 ## Development
 
 ### Setting up your development environment