adds contributing

CONTRIBUTING.md

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+## Contributing
+
+First off, thank you for considering contributing to this repo. It's people
+like you that make this material such a great tutorial.
+
+### 1. Found a bug, have a question or want to add a new feature
+
+If you've noticed a bug, a feature or have a question go ahead and [open an issue](https://github.com/leandromoreira/digital_video_introduction/issues/new)!
+
+### 2. Fork & create a branch
+
+> * **Ensure the bug was not already reported** by [searching all issues](https://github.com/leandromoreira/digital_video_introduction/issues?q=).
+
+If this is something you think you can fix, then
+[fork digital_video_introduction](https://help.github.com/articles/fork-a-repo)
+and create a branch with a descriptive name.
+
+A good branch name would be (where issue #325 is the ticket you're working on):
+
+```sh
+git checkout -b 325-add-japanese-translations
+```
+
+### 3. Implement your fix or feature
+
+At this point, you're ready to make your changes!
+
+### 4. Make a Pull Request
+
+At this point, you should switch back to your master branch and make sure it's
+up to date with Active Admin's master branch:
+
+```sh
+git remote add upstream git@github.com:leandromoreira/digital_video_introduction.git
+git checkout master
+git pull upstream master
+```
+
+Then update your feature branch from your local copy of master, and push it!
+
+```sh
+git checkout 325-add-japanese-translations
+git rebase master
+git push --set-upstream origin 325-add-japanese-translations
+```
+
+Finally, go to GitHub and
+[make a Pull Request](https://help.github.com/articles/creating-a-pull-request)
+:D
+
+### 5. Merging a PR (maintainers only)
+
+A PR can only be merged into master by a maintainer if:
+
+* It was reviewed.
+* It is up to date with current master.
+
+Any maintainer is allowed to merge a PR if all of these conditions are
+met.
+
+PS: this contributing text was inspired by active admin contributing.