Fix readme for go get git-checkout-history

This is what I needed to do in order to checkout this repository locally.

I also used nix-shell, so I set my GOPATH to ~/go from inside the nix-shell environment.

This functionality has been implemented as of v0.2.0. It is no longer
an issue.

When getting a list of branches but the file doesn't exist, print a
custom error message that informs users that it's necessary to run
`git-checkout-store` before `git-checkout-history` will be able to do
anything.

Use a different history list for each repo.

Now the YAML file is organised in this way:

    /path/to/repo:
      - branch name
      - branch name
    /path/to/another/repo:
      - branch name

A note and installation instructions for shell completion of
`git-checkout-store`.

Add a patch file that can be used on git's git-completion.bash file.
This gives `git-checkout-store` the same shell completion provided to
`git checkout`.

For those accustomed to git shell completion, this should make
`git-checkout-store` easier to use.