README.md: Getting started change git clone error

[Frank Becker]

Please change the

git clone [e-mail]:marcelbirkner/docker-ci-tool-stack.git

to

git clone https://github.com/marcelbirkner/docker-ci-tool-stack.git

or you get the following error:
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

[Fede Diaz]

You need to add ssh-key to your account in order to use the first one.

[Marcel Birkner]

@BeckerFrank / @nordri
Thanks for the input. I added your comments to the README