Add git guide

guides/git.md

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+## Working with git
+
+## Syncing your fork back to the remote master
+
+After you submitted your assignment and received your feedback, chances are it was closed without
+being merged into the remote master (the official course repository). You now have a fork that is
+not in the same state as the official copy and you need to sync it back for the next assignment. If
+you don't, future pull requests will include the previous assignments and will create a mess.
+
+There are two main ways to get back to sync and one way to avoid this problem in the future:
+
+### Delete your fork and fork again
+
+This is the simplest. You just go to your fork on GitHub, go to **settings** and at the bottom
+click on **Delete this repository**. Once deleted, go back to
+[the official repository](https://github.com/hueniverse/hueniversity) and fork it again. All set.
+Remember to also delete your local directory and then to clone your new fork all over again.
+
+### Replace your master branch with the remote branch.
+
+```
+git remote add upstream https://github.com/hueniverse/hueniversity
+git fetch upstream
+git branch backup
+git checkout upstream/master -B master
+git push origin master --force
+```
+
+You are now back in sync and can start the next assignment
+
+### Avoid this problem in the first place
+
+Once your fork is in sync (using one of the two methods above), do not make changes directly to
+your master branch. Instead, make a local branch for the current assignment:
+
+```
+git branch assignment1
+git checkout assignment1
+```
+
+Make your changes as instructed by the assignment and save them.
+
+```
+git add .
+git commit -m 'my changes'
+git push origin assignment1
+```
+
+When done, go to your fork on GitHub, switch to the assignment branch using the button on the top
+left, right above the file list, and then initiate a pull request.