add rfc template

[yoshuawuyts]

Description

This adds an RFC template to Tide, similar to rustwasm's RFC template.

Motivation and Context

When making changes to an API it can be useful to get consensus on the design first in a structured manner before moving to implementation. The Rust project has proven that filing RFCs has been a a good strategy to discuss these kinds of changes, and we're seeing a similar process work well for rustwasm/gloo and crossbeam.

I've chosen to add the RFC template directly to Tide so we can discuss changes in the same repo. Having a separate RFC directory could be another option, but I feel that could be something we can move to at a later stage if we think that's desirable. The main goal for now is to provide us with a more structured way of proposing API changes, for which this should be the right way to go.

How Has This Been Tested?

No tests needed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[xenith]

This has received no objections since being raised, going to go ahead and merge so we can build on top of it.