Minor revisions in development practices.

Despite the diffs looking like I ripped out and replaced huge chunks of text, these are mostly minor revisions like removing unnecessary instances of "the" or "that" and re-arranging clauses to be as direct as possible. I kept the convention to wrap at 80 chars per line, causing small changes to cascade through paragraphs.

Because the resultant diff is not at all helpful, here are the notable changes above and beyond minor tweaks:

* Add "powerful, diverse image processing tools" to the stated purpose of scikit-image (images not previously mentioned)
* Add Oxford commas
* Replace "bug reports" from the Google Groups purpose list with "community development" - I believe we want to encourage use of GitHub issues for bug reports, rather than the mailing list. "User support" remains.
* Add a sentence noting the modular, functional design lowers the barrier for new contributors to hack in additional features, as package-wide expertise is not required
* Note code coverage "continues to rise," as this is a medium-term goal
* Specify and reference the code is analyzed by `flake8` for PEP8 compliance instead of "several tools" (alternative: could remove `flake8` and just note the code is analyzed for PEP8 compliance if we may use alternatives in future)
* Add a reference for the scikit-image GitHub repository
* Clarify the distinction between minor and major releases
* Add a final sentence about deprecation warnings across a full major release for API-breaking changes.

The existing development practices section is very well done. I believe these relatively subtle changes are minor but significant improvements.