Add discussion of variable definition directives (#103)

* Add myself to Attendees

* Add discussion of variable definition directives

See graphql/graphql-spec#510 for background: basically, is the API correct. It matches the API for directives on field argument definitions, and is (I believe) the most glaring "hole" in where we allow you to add directives.

agendas/2018-09-20.md

@@ -17,6 +17,7 @@ Name                 | Organization  | Location
 -------------------- | ------------- | ----------------------
 Lee Byron            | Robinhood     | Menlo Park, CA
 Michael Staib        | ChilliCream   | Zurich, Switzerland
+Matt Mahoney         | Facebook      | New York, NY
 
 <small>\*: willing to take notes (eg. <em>Joe Montana*</em>)</small>
 
@@ -26,6 +27,7 @@ Michael Staib        | ChilliCream   | Zurich, Switzerland
 1. Introduction of attendees
 1. Review agenda (5m)
 1. Determine volunteers for note taking (5m)
+1. Discussion of [Variable Definition Directives](https://github.com/facebook/graphql/pull/510) (5m)
 
 ## Agenda and Attendee guidelines