Add placeholders for new sections (#616)

* Add placeholders for new sections

* contributing note

Author: binaryseed

rfcs/InputUnion.md

@@ -7,6 +7,25 @@ This document attempts to bring together all the various solutions that have bee
 
 From that shared understanding, this document can then evolve into a proposed solution.
 
+### Contributing
+
+To help bring this idea to reality, you can contribute through two channels:
+
+* Discuss the idea in this Github Issue:
+  * https://github.com/graphql/graphql-spec/issues/617
+* Submit PRs to this RFC document
+  * https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md
+
+## Problem Statement
+
+TODO:
+* [ ] Describe at a high level the problem that Input Union is trying to solve.
+
+## Use Cases
+
+TODO:
+* [ ] Collect use cases from the community's related Issues / PRs.
+
 ## Possible Solutions
 
 Categories: