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

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](https://stackoverflow.com/q/606483)
- [Trouble setting up git with my GitHub Account error: could not lock config file](https://stackoverflow.com/q/14959972)
- [How to fix Git for Windows error: “could not lock config file C:/file/path/to/.gitconfig: File exists”?](https://stackoverflow.com/q/55327408)
- [git index.lock File exists when I try to commit, but cannot delete the file](https://stackoverflow.com/q/9282632)
- [Change the location of the ~ directory in a Windows install of Git Bash](https://stackoverflow.com/q/32232978)
- [Changing .gitconfig location on Windows](https://stackoverflow.com/q/4050905)