add contributing info

CONTRIBUTING.md

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+# How to contribute
+
+If you would like to contribute to the project please follow the guidelines set out below. Keep in mind that they are not here to make your contribution a painful experience, but to simplify our jobs looking through hundreds of issues and pull requests (making it a 30 minute task instead of a 4 hour job!)
+
+## Pull Request
+
+Pull Request for new features, bugs or translations are often appreciated. However please follow the following guidelines to save as much time as possible for the maintainer.
+
+- __Make your commit message as descriptive as possible.__ Include as much information as you can. Explain anything that the file diffs themselves won’t make apparent.
+- __Document your pull request__. Explain your fix, link to the relevant issue. A pull request without any comment will get closed.
+- __Consolidate multiple commits into a single commit when you rebase.__ If you’ve got several commits in your local repository that all have to do with a single change, you can squash multiple commits into a single, clean, descriptive commit when using git-rebase. When you do, good karma is yours.
+- __Make sure the target of your pull request is the relevant dev branch__. Most of bugfix or new feature should go to the `dev` branch.
+- __Include only commits fixing a specific issue__. If your pull request has unrelated commit, it will get closed.
+
+### UI changes
+
+More to come on the official release
+
+## Report a bug
+
+Before reporting any issues, please use the search tools to see if someone filed the same bug before.
+
+When creating a new issue make sure to include the following:
+- Version of Soundnode App used. Are you running from source? Which revision? Are you using a released build? Which release?
+- Your environment. What is your operating system? 32 or 64 bits?
+- Step to reproduce. Even if the step is only to open the app, __include it!__ Include the actual result and what you expected.
+- Messages you get when running from console with the `--debug` parameter.
+- A screenshot of any visual bug.
+
+Here is what a great bug report would look like:
+```
+Song not playing
+
+Version: Release 0.2.7 for Windows
+Downloaded from: soundnodeapp.com
+OS: Windows 7 64bits
+
+How to reproduce:
+ - Open Soundnode App
+ - Click on the `first Hip Hop` song in `stream` category
+ - Click "play"
+ - Wait for song to start
+Actual result:
+ - the song doesn't start
+ - not show the current song being play
+
+Console output:
+[6239:0317/031639:INFO:CONSOLE(0)] "event.returnValue is deprecated. Please use the standard event.preventDefault() instead.", source:  (0)
+...
+```
+
+## Feature suggestions
+
+Create a issue with name [FEATURE SUGGESTION]