Formatting: remove extraneous newlines.

Rely on reasonable text editors to wrap paragraphs to the editor window.  GitHub's web-based editor does this.
Emphasize contributor license agreement.
Add advice re communicating.
Trivial: fix typo: "committing" --> "committing".
Add advice about coding and collaborating.
Pull requests and JIRA as communication opportunities.
Further justify having a JIRA issue for the change.
Trivial: fix typo "inclusing" --> "including".
Standardize on JIRA rather than Jira.
Trivial: blank lines after section headings.
Add links to up-dev@ and up-user@ email lists.
Link code conventions and architecture pages.
Clarify suggestion to collaborate in feature branch on public fork.
Add guidance on Pull Request naming and on linking JIRA from PR.
Merge pull request #461 from apetro/UP-4148

UP-4148 Update CONTRIBUTING.md