Add top matter, clarify that feature gate would be removed

Author: nikomatsakis

active/0000-defaulted-type-params.md

@@ -1,3 +1,7 @@
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
 # Summary
 
 Rust currently includes feature-gated support for type parameters that
@@ -19,6 +23,8 @@ parameters:
 Points 2 and 4 extend the current behavior of type parameter defaults,
 aiming to address some shortcomings of the current implementation.
 
+This RFC would remove the feature gate on defaulted type parameters.
+
 # Motivation
 
 ## Why defaulted type parameters
@@ -95,6 +101,10 @@ older behavior.
 
 # Detailed Design
 
+## Remove feature gate
+
+This RFC would remove the feature gate on defaulted type parameters.
+
 ## Type parameters with defaults
 
 Defaults can be placed on any type parameter, whether it is declared