Add a template RFC

Author: brson

0000-template.md

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+# Summary
+
+One para explanation of the feature.
+
+# Motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+# Detailed design
+
+This is the bulk of the RFC. Explain the design in enough detail for somebody familiar
+with the language to understand, and for somebody familiar with the compiler to implement.
+This should get into specifics and corner-cases, and include examples of how the feature is used.
+
+# Alternatives
+
+What other designs have been considered? What is the impact of not doing this?
+
+# Unresolved questions
+
+What parts of the design are still TBD?