Updates for relative links and being clear on version dependencies

[cholmes]

Related Issue(s): #55

Proposed Changes:

1. Add note about properly checking out submodules
2. Update stac-spec links to be relative
3. Make clear the features API dependency
4. Added a PROCESS.md to have a place to put reminders for release.
5. Add what updates are needed for a 'release' (update submodule). Ended up making it so you don't need to update
   the stac-spec version - just instructed people to check the CHANGELOG of the submodule to see.

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• OpenAPI files only: I have run npm run generate-all to update the generated OpenAPI files.

[m-mohr]

I'm wondering whether we should have a central place for things that are common across stac-spec and stac-api-spec. It's always a pain to keep things aligned...

[cholmes]

I'm open to it, but not sure how it would work. I feel like it either is harder for a new user to understand (go look at this repo to find out x, y & z), or it's some complicated technical thing (though maybe submodules come close)?

I sorta figured the release processes would also diverge some.

[m-mohr]

Seems like a good start. Maybe add a CHANGELOG entry for the new definition that this API version only supports spec 0.9 up to 2 (exclusive)?