Update release process docs

[jmbowman]

Summary of changes

• Updated the release process docs to account for towncrier
• Removed blank lines between the non-labeled categories of changes in a release
• Added changelog.d entries for PRs going into the next release which preceded the addition of towncrier support

Currently the changelog entry for the next release comes out like this:

v39.2.0
-------

* #1359: Support using "file:" to load a PEP 440-compliant package version from
  a text file.
* #1360: Fixed issue with a mismatch between the name of the package and the
  name of the .dist-info file in wheel files
* #1365: Take the package_dir option into account when loading the version from
  a module attribute.
* #1353: Added coverage badge to README.
* #1356: Made small fixes to the developer guide documentation.
* #1357: Fixed warnings in documentation builds and started enforcing that the
  docs build without warnings in tox.
* #1376: Updated release process docs.
* #1352: Added ``tox`` environment for documentation builds.
* #1354: Added ``towncrier`` for changelog managment.
* #1355: Add PR template.
* #1368: Fixed tests which failed without network connectivity.
* #1369: Added unit tests for PEP 425 compatibility tags support.
* #1372: Stop testing Python 3.3 in Travis CI, now that the latest version of
  ``wheel`` no longer installs on it.

Pull Request Checklist

• News fragment added in changelog.d. See documentation for details

[jaraco]

Thanks for putting this together.