Add an RFC template

rfcs/0000-template.rst:
Add a basic RFC template that provides a structure with Summary,
Motivation, Specification, Drawbacks and Unresolved Questions.

rfcs/0000-template.rst

+=====
+Title
+=====
+
+- Date proposed: yyyy-mm-dd
+- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/0000
+  **update this number after RFC merge request has been filed**
+
+Summary
+-------
+
+Short description, 1-2 sentences.
+
+Motivation
+----------
+
+Introduce the topic. If this is a not-well-known section of Arch Linux, a
+detailed explanation of the background is recommended.
+
+Some example points of discussion:
+
+- What specific problems are you facing right now that you're trying to address?
+- Are there any previous discussions? Link to them and summarize them (don't
+  force your readers to read them though!).
+- Is there any precedent set by other software? If so, link to resources.
+
+Specification
+-------------
+
+A concrete, thorough explanation of what is being planned.
+
+Drawbacks
+---------
+
+Carefully consider every possible objection and issue with your proposal. This
+section should be updated as feedback comes in from discussion.
+
+Unresolved Questions
+--------------------
+
+Are there any portions of your proposal which need to be discussed with the
+community before the RFC can proceed? Be careful here -- an RFC with a lot of
+remaining questions is likely to be stalled. If your RFC is mostly unresolved
+questions and not too much substance, it may not be ready.
+
+Alternatives Considered
+-----------------------
+
+A list of alternatives, that have been considered and offer equal or similar
+features to the proposed change.