Add an issue template for API proposals

.github/ISSUE_TEMPLATE/api-proposal.md

@@ -0,0 +1,31 @@
+---
+name: API Proposal
+about: Propose a change to the public API surface.
+title: "[API Proposal] Your title goes here"
+labels: api-suggestion
+assignees: ''
+
+---
+
+# Background and Motivation
+
+Please provide a thorough explanation of what necessitates the additional API.
+
+# Proposed API
+
+Please provide the precise public API signature diff that you are proposing. If augmenting existing API signatures this should be expressed using diff blocks, for example
+```diff
+namespace System.Collections.Generic
+{
+-    public class HashSet<T> : ICollection<T>, ISet<T> {
++    public class HashSet<T> : ICollection<T>, ISet<T>, IReadOnlySet<T> {
+     }
+```
+
+# Usage Examples
+
+Please provide code examples that highlight how the proposed API additions are meant to be consumed.
+
+# Risks
+
+Please mention any risks that to your knowledge the API proposal might entail, such as breaking changes, performance regressions, etc.