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

@@ -0,0 +1,40 @@
+---
+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) -->