Add docs for filtering and custom function plugins

PLUGINS.md

@@ -27,4 +27,18 @@ Inspect external instances (their ID and parsed XML) after parsing or provide cu
 ### API
 - `ExternalInstanceParser#addFileInstanceParser`
 
-The default `ExternalInstanceParser` can be overridden by creating an implementation of `ExternalInstanceParserFactory` and calling `XFormUtils.setExternalInstanceParserFactory` with it.
+The default `ExternalInstanceParser` can be overridden by creating an implementation of `ExternalInstanceParserFactory` and calling `XFormUtils.setExternalInstanceParserFactory` with it.
+
+## Function
+
+Add custom functions that can be called from XPath.
+
+### API
+- `FormEntryController#addFunctionHandler`
+
+## Predicate evaluation
+
+Add custom strategies for filtering nodes for predicate expressions.
+
+### API
+- `FormEntryController#addFilterStrategy`