Update readme instructions with `git fetch upstream` if using a fork

[mguaypaq]

A bit, yes... :-D I have the data updated, so we can close this issue. Only question remain, if my case was just too specific or if it would be worth to add the potential neccesity of the git fetch upstream command into README.md

Originally posted by @renelabounek in #125 (comment)

When working though a github fork of this repo, the git-annex branch (which contains the location information for file contents used by git annex get) doesn't automatically get updated in the fork, so it's important to get it from the git-annex branch of this repo.