docs: Setup template for release note process

# Pending release notes directory

This directory contains release note entries that have not been merged

into the main release-notes.md document.

Generally a release note entry should be created for changes that:

- Fix bugs in the code.

- Implement new features.

- Change existing behavior.

Release notes are generally not needed for:

- Some documentation improvements.

- Strictly internal changes to the code that won't be visible to users

  of the code.

## Release note format

Release notes are included in files under this `docs/release-notes.d`

directory and have a name of `*.md`.  They will be included in the

`release-notes.md` file, and should be formatted as a Markdown list

entry.  (A script will be developed to collect these, ordered by when

the commits were added to the tree.)

Choose a filename that is related to what this change does.  The names

are not used for anything in particular, but to keep the files

distinct so that there isn't a concern with merge conflicts as

different pull requests merge in different orders.

to describe the release. This should be a high-level description of

the changes, not a list of the git commits.

Provided that changes going into the release have followed the

contribution guidelines, this should mostly consist of collecting the

various snippets in the `docs/release-notes.d` directory.  After

incorporating these snippets into the release notes, the snippet files

should be removed to ready the directory for the next release cycle.

## Release candidates

Before each release, tags are made (see below) for at least one