Update developer docs in support of July Hackathon

[mr-c]

• [ ] Outline current documentation
• [ ] Modify outline to show proposed changes using https://github.com/ged-lab/khmer/issues/321#issuecomment-37586371 and other items from below

Evolvability tasks #299 Changeability tasks #298 doc contributor policies #155 Add semantic versioning/backwards compatibility requirements to checklist #373 Analysability tasks #297 Supportability tasks #296 Copyright tasks #292 transfer TODOs in docs to github issues #205 Documentation tasks #289 investigate diff-cover #251 Community tasks #294 Learnability tasks #290 Understadability tasks #288 Put in place guidelines around last-minute changes in releases #372 Reduce memory requirements for many unit tests #444 Governance tasks #293 Add Torsten's rant into our docs re command line stuff #209 doc/guide.txt needs editing #202 Figure out how to document and channel contributors #145 remaining bits from https://github.com/ged-lab/khmer/issues/321 Make a screencast of what successful clone, compile + tests looks like #152

[ctb]

We should also add information on how to run tests: #463

[ctb]

Also see #464