Update README.textile

Add upgrade command to docs. I canceled the prompt once and went hunting for this so I could upgrade.
Merge pull request #1870 from mfrobben/patch-1

Update README.textile
Add theme 'illenseer'
Fix git deleted status
Fix ahead status and add stashed status
Add parse_git_dirty explicitly
Fix git status
Use 'staged' instead of 'added'
Refactor if-then-else constructs
Fix theme, remove unnecessary ${parse_git_dirty}