(maint) Update README.md for support clarification

This commit gives our README.md a little bit of love:

- adds verbiage about support lifecycle
- moves HTTP API docs up near the rest of the docs
- updates the link to the ticket tracker now that we're on JIRA
Fix trailing whitespace, add EOL clarification
Wording tweaks from review feedback
Merge pull request #2316 from ahpook/clean-master

(maint) Update README.md for support clarification