Create PR template for the nmdc-schema repository

[eecavanna]

I recommend using the raw Markdown of the current nmdc-runtime PR template as a starting point, as it has some handy HTML comments in it that are visible to the person filling out the template, but not to the person reading the resulting PR description.

[eecavanna]

I volunteered to make one and present it to the group for discussion.

[eecavanna]

A quote from a messages I wrote on Slack today:

I see the version naming guidelines in MAINTAINERS.md. For starters, I would like to have a section in the PR template where the author marks a checkbox indicating which of those three categories the changes fall into (or the PR template instructs them to apply a specific label to the PR).