ADR 072: Re-instate a request-for-comments archive. (#6851)

This ADR restores a variation of the old Request for Comments documentation
that we previously used. The proposal differs from the original formulation,
and does not replace ADRs.

docs/rfc/README.md

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+---
+order: 1
+parent:
+  order: false
+---
+
+# Requests for Comments
+
+A Request for Comments (RFC) is a record of discussion on an open-ended topic
+related to the design and implementation of Tendermint Core, for which no
+immediate decision is required.
+
+The purpose of an RFC is to serve as a historical record of a high-level
+discussion that might otherwise only be recorded in an ad hoc way (for example,
+via gists or Google docs) that are difficult to discover for someone after the
+fact. An RFC _may_ give rise to more specific architectural _decisions_ for
+Tendermint, but those decisions must be recorded separately in [Architecture
+Decision Records (ADR)](./../architecture).
+
+As a rule of thumb, if you can articulate a specific question that needs to be
+answered, write an ADR. If you need to explore the topic and get input from
+others to know what questions need to be answered, an RFC may be appropriate.
+
+## RFC Content
+
+An RFC should provide:
+
+- A **changelog**, documenting when and how the RFC has changed.
+- An **abstract**, briefly summarizing the topic so the reader can quickly tell
+  whether it is relevant to their interest.
+- Any **background** a reader will need to understand and participate in the
+  substance of the discussion (links to other documents are fine here).
+- The **discussion**, the primary content of the document.
+
+The [rfc-template.md](./rfc-template.md) file includes placeholders for these
+sections.
+
+## Table of Contents
+
+<!-- - [RFC-NNN: Title](./rfc-NNN.title.md) -->

docs/rfc/rfc-template.md

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+# RFC {RFC-NUMBER}: {TITLE}
+
+## Changelog
+
+- {date}: {changelog}
+
+## Abstract
+
+> A brief high-level synopsis of the topic of discussion for this RFC, ideally
+> just a few sentences.  This should help the reader quickly decide whether the
+> rest of the discussion is relevant to their interest.
+
+## Background
+
+> Any context or orientation needed for a reader to understand and participate
+> in the substance of the Discussion. If necessary, this section may include
+> links to other documentation or sources rather than restating existing
+> material, but should provide enough detail that the reader can tell what they
+> need to read to be up-to-date.
+
+### References
+
+> Links to external materials needed to follow the discussion may be added here.
+>
+> In addition, if the discussion in a request for comments leads to any design
+> decisions, it may be helpful to add links to the ADR documents here after the
+> discussion has settled.
+
+## Discussion
+
+> This section contains the core of the discussion.
+>
+> There is no fixed format for this section, but ideally changes to this
+> section should be updated before merging to reflect any discussion that took
+> place on the PR that made those changes.