Reorganized “What makes a good CHANGELOG?”

- Reordered bullets to go from general ▶︎ specific (that is, overall/document guidelines ▶︎ per-version guidelines)
- Edited phrasing of bullets for clarity (and a slightly more “instructions”- or “checklist”-like tone)
Minor markup tweak
Merge pull request #16 from Zearin/README-tweaks

Readme tweaks