UP-4148 Update CONTRIBUTING.md

See [UP-4148](https://issues.jasig.org/browse/UP-4148).  This is a retry of PR #347 dropping a change to the recommended branch naming convention.

Updates `CONTRIBUTING.md` to

 * Emphasize the necessity of the Individual Contributor License Agreement (ICLA).
 * Suggest communicating about proposed changes via `uportal-dev@` and `uportal-user@`.
 * Request that code consider style conventions and uPortal architecture.
 * Suggest sharing and collaborating in a public feature branch in advance of pull request.
 * Standardize on "JIRA" vs "Jira".
 * Fix "inclusing" and "commiting" typos.
 * Remove extraneous within-paragraph newlines.  Add newlines after section headings.

The change to a branch naming convention that would supplement the bare issue identifier in the name with a hint about what the branch is about (`UP-4148-contributing` vs `UP-4148`) is dropped because there had been [concern that long branch names might be annoying](https://github.com/Jasig/uPortal/pull/347#issuecomment-46590047).