Add instructions for installing Git in a Windows environment with network drives

[parkerbxyz]

If you are in a corporate Windows environment with a mapped network drive and modified HOME path, chances are you will install Git onto the mapped network drive. This will cause Git to run verrrrrry slowly because each command will be run on a remote server instead of locally. This also often causes .gitconfig permissions problems.

We should add information about how to install Git to your local drive if you are in an environment like this.

https://github.com/githubtraining/training-manual/blob/master/docs/01_getting_ready_for_class.md

Potentially helpful references:

• What is the meaning of these Windows Environment variables: HOMEDRIVE, HOMEPATH, HOMESHARE, USERPROFILE
• Trouble setting up git with my GitHub Account error: could not lock config file
• How to fix Git for Windows error: “could not lock config file C:/file/path/to/.gitconfig: File exists”?
• git index.lock File exists when I try to commit, but cannot delete the file
• Change the location of the ~ directory in a Windows install of Git Bash
• Changing .gitconfig location on Windows