Document that end users shouldn't be running `git` commands manually where nbgitpuller clones

[yuvipanda]

nbgitpuller expects to have full ownership of the cloned git repository. If the user is manually running git commands there, it will eventually break. This is an essential complexity of nbgitpuller - its goal is to abstract away git usage from the end user, so it makes assumptions about how git is used. Many things a user can do (particularly switching branches) breaks these assumptions.

If the user wants to run git commands locally, they should make a clone with git clone.

Definition of done

• This information is prominently documented