Summarize EML 2.2 changes

[mbjones]

Write overview of 2.2 changes.

[mbjones]

Should be in a markdown document in docs parallel to the prior summary changes from previous releases.

[mbjones]

Here's an example from a previous release: https://github.com/NCEAS/eml/blob/BRANCH_EML_2_2/docs/eml-210info.md

[mbjones]

@twhitaker drafted this, which was merged from PR #313. Thanks @twhitaker. Still need to:

• Edit the document addressing Tim's questions
• Add Tim to the contributor lists

[mbjones]

Reviewed and finalized the changes to 2.2.0 document.