Avoid hardcoding pip cache directory

Hardcoding the cache values in 221954d (i.e., `--download-cache
$VIRTUALFISH_HOME/_pip_cache`) will likely override the user's pip
configuration settings. In my case, I have the following in
`config.fish`:

    set -x PIP_DOWNLOAD_CACHE "$HOME/.pip/cache"

If I already have a metric ton of cached packages in
`$HOME/.pip/cache` (and I do), why would I want or need another
cache created in `$VIRTUALFISH_HOME/_pip_cache`?

(In case you're wondering, "Maybe Virtualfish should look for these
environment variables, and if they aren't found, fall back to
defaults such as `$VIRTUALFISH_HOME/_pip_cache`?" ... that would
still potentially cause problems because pip configuration can also
be stored in `~/.pip/pip.conf`.)

This commit removes the hardcoded values and instead provides
instructions in the README that explain how to use pip's cache and wheel
settings to speed up global requirements installations.