Add note about `VIRTUAL_ENV_DISABLE_PROMPT`

Fixes #5

README.md

@@ -41,6 +41,7 @@ This theme is based loosely on [agnoster][agnoster].
  * User@Host (unless you're the default user)
  * Current RVM or rbenv (Ruby) version
  * Current virtualenv (Python) version
+     * _If you use virtualenv, you will probably need to disable the default virtualenv prompt, since it doesn't play nice with fish: `set -x VIRTUAL_ENV_DISABLE_PROMPT 1`_
  * Abbreviated parent directory
  * Current directory, or Git or Mercurial project name
  * Current project's repo branch ( master) or detached head (➦ d0dfd9b)