rework MELPA install

- more clarity on stable package source URL
- more clarity on adding package source
- add line breaks for 80ch

- one (M)ELPA heading
- short list overview
- immediately followed by exact code syntax
- followed by note regarding stable vs regular MELPA
- next heading: Download latest release
Merge pull request #146 from rickhull/master

Rework MELPA install section of README.md