documented previously undocumented plugin methods

docs/plugins.md

@@ -1,4 +1,6 @@
-# Plugins
+# Legacy plugins
+
+Note: this is a legacy subsystem, being replaced with a new, composer-based one.
 
 Psalm can be extended through plugins to find domain-specific issues.
 
@@ -12,9 +14,13 @@ class SomePlugin extends \Psalm\Plugin
 return new SomePlugin;
 ```
 
-`Psalm\Plugin` offers two methods that you can override:
+`Psalm\Plugin` offers six methods that you can override:
  - `afterStatementsCheck` - called after Psalm evaluates each statement
  - `afterExpressionCheck` - called after Psalm evaluates each expression
+ - `afterVisitClassLike` - called after Psalm scans(?) a class-like (class,interface,trait)
+ - `afterClassLikeExistsCheck` - called after Psalm analized a reference to a class-like
+ - `afterMethodCallCheck` - called after Psalm analized a method call
+ - `afterFunctionCallCheck` - called after Psalm analized a function call
 
 An example plugin that checks class references in strings is provided [here](https://github.com/vimeo/psalm/blob/master/examples/StringChecker.php).