* add subsections to "submit a PR" section
this allows the reader to visually separate the instructions and makes the longer git-command section less prone to trigger a TLDR (heh) when reading this part.
also, line wrapping tweaks, split a paragraph into two, and reword a couple passages for clarity
* compact the list of git commands
* CONTRIBUTING: greatly simplify the PR instructions
plus some wording tweaks here and there
* improve instructions to use tldrl
* brace expansion is not as common in the wild
removing it keeps the examples of unix features to a manageable/readable 3
* use actual commands in the git section
* use only one-liners on the git section
* add subsection header for commit message
* slight reword of licensing note
- merge do's and don'ts, since they had mostly the same advice
- move token formatting info to the guideline section and the markdown section, as appropriate
- tweak spelling and fix a couple typos
- fix options vs. values in the formatting example, and add a token there.