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
virtualenvwrapper plugin cleanup

* removes cd override by using chpwd_functions
* removes subshell call to which by using $+commands array and
  c param expansion to find in PATH
* zsh love!
virtualenv cleanup: replaces subshell with prompt expansion.

* :t parameter expansion returns the last portion of the path,
  equivalent to basename. I <3 zsh.
* adds comments for the VIRTUAL_ENV_DISABLE_PROMPT,
  used by virtual_env activate

See also:
  http://zsh.sourceforge.net/Doc/Release/Expansion.html#Parameter-Expansion
Escape both % and $ in the command line

Fixes issue #1884
Merge pull request #1878 from spazm/virtualenv-cleanup

virtualenv cleanup: replaces subshell with prompt expansion.
Merge pull request #1877 from spazm/virtualenvwrapper-cleanup

virtualenvwrapper plugin cleanup
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh
Add new plugin for homebrew installed version of postgres
debian plugin: ignore alias in sudo/aptitude check
Merge pull request #1893 from jeremydt/postgres

Add new plugin for homebrew installed version of postgres
Merge pull request #1894 from codewhale/master

debian plugin: ignore alias in sudo/aptitude check
Merge pull request #1885 from eakret/termsupport-fix

Escape both % and $ in the command line
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh