Improve the “Contribute Code” docs
Improve the “Contribute Code” docs

- Fix a bunch of spelling, grammar, and punctuation issues
- Remove the `> ` in the command-line examples so they’re easier to copy + paste
- Organize the file structure alphabetically
- Add a note about manually upgrading the code from the DoneJS plugin generator to CanJS 3

Closes #2463

gsmeets/canjs,bitovi/canjs,bitovi/canjs,gsmeets/canjs,bitovi/canjs,bitovi/canjs,gsmeets/canjs