Closing issues cross-repo now works

See https://github.com/blog/1439-closing-issues-across-repositories
Add missing keywords that can close an issue
Update fork image
Update forking and cloning instructions

- Reduce to 3 steps
- Explain better what's happening
- Remove mentions of experimental since that branch no longer exists
Remove Git Philosophy section

Was out of place. I'll probably add some of those tips to Best Practices.
Improve forking and cloning instructions

- Fix a typo
- Be more clear about what a fork is
- Replace `git fetch upstream` with `git remote -v`.
  It's unlikely they need to fetch at that time, but making sure
  they set up the remote correctly is important.
Rewrite Basic Workflow

- Instructions are much more detailed and explained now
- Command-line examples are given for appropriate steps
- I threw in some good practices as well, so they learn them early