Split 06-collaborating in two files

[iglpdc]

When reviewing 06-collaborating.md, I noticed that it has two different parts. The first deals on how to set up a remote repo in Github for your local repo, and the second on how to add collaborators and have them cloning your repo. I'd like to split it in two shorter topics.

An advantage of this is that i t would be decouple the two parts, so we have several alternatives for the collaboration section (as is, doing PRs, role-play between instructors.)

If people is in favor of that, I could split it for 5.3 and we'll consider alternative collaboration sections for future releases.

What do you think?

[iglpdc]

Fixed in #116.