Cors practice

[cholmes]

Related Issue(s): #888

Proposed Changes:

1. Added a section to the best practices about enabling CORS.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR Add recommendation to enable CORS stac-api-spec#99 to track the change.

[cholmes]

Oh yeah, meant to comment that I'm not a CORS expert, so please do sound in if anything sounds a bit off, or if there's tweaks to make.