PR Contribution - Adding GIT init instructions (First install)

[bashfulrobot]

Hi there, as discussed on #143 I plan to make a PR for some additional instructions in the README file on how to init the chezmoi source the first time with git.

I was reading the contributing guide, and it asks that an issue is opened up to discuss.

I'm going to have to go back and fix my commit messages to match your format (I had done the work prior to reading the contributing guide).

I have added sub-heading to format in such a way that someone else more familiar with the supported VC systems could also add in the future.

i.e.

## Managing your `~/.chezmoi` directory with version control

....

### Setting Up the Initial VC Repo

Currently, this document only covers the GIT VC system.

#### GIT

....

#### Other VC1

....

#### Other VC2

....

##### Optional Steps - Adding to an existing GitHub Repo

....

...

What other aspects would you like to discuss?

[twpayne]

Thanks, #150 merged.