docs: add documentation for onProtoPoisoning option in README

Author: bjohansebas

README.md

@@ -116,6 +116,10 @@ The `verify` option, if supplied, is called as `verify(req, res, buf, encoding)`
 where `buf` is a `Buffer` of the raw request body and `encoding` is the
 encoding of the request. The parsing can be aborted by throwing an error.
 
+##### onProtoPoisoning
+
+Defines what action must be taken when parsing a JSON object with `__proto__`
+
 ### bodyParser.raw([options])
 
 Returns middleware that parses all bodies as a `Buffer` and only looks at