docs(contributing): add contributing documentation

CONTRIBUTING.md

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+# How to contribute
+
+We love you to contribute to this project by filing bugs, helping others on the [issue tracker](https://github.com/bpmn-io/bpmn-js/issues), by contributing features/bug fixes through pull requests or by helping out in our [forums](https://forum.bpmn.io/).
+
+## How to start with bpmn-js
+
+The bpmn-js runs on top of the diagram-js master branch. The following code snippet sets up both libraries linking diagram-js to bpmn-js.
+    
+    mkdir bpmn.io
+    cd bpmn.io
+
+    git clone git@github.com:bpmn-io/diagram-js.git
+    (cd diagram-js && npm i)
+
+    git clone git@github.com:bpmn-io/bpmn-js.git
+    (cd bpmn-js && npm install && npm link ../diagram-js)
+    
+    // Run the test suite
+    grunt
+    
+    // Running the test suite with every file change
+    TEST_BROWSERS=(Chrome|Firefox|IE) grunt auto-test
+
+## Working with issues
+
+We use our [issue tracker](https://github.com/bpmn-io/bpmn-js/issues) for project communication.
+When using the issue tracker,
+
+* Be descriptive when creating an issue (what, where, when and how does a problem pop up)?
+* Attach steps to reproduce (if applicable)
+* Attach code samples, configuration options or stack traces that may indicate a problem
+* Be helpful and respect others when commenting
+
+Create a pull request if you would like to have an in-depth discussion about some piece of code.
+
+## Creating pull requests
+
+We use pull requests for feature discussion and bug fixes. If you are not yet familiar on how to create a pull request, [read this great guide](https://gun.io/blog/how-to-github-fork-branch-and-pull-request).
+
+Some things that make it easier for us to accept your pull requests
+
+* The code adheres to our conventions
+    * spaces instead of tabs
+    * single-quotes
+    * ...
+* The code is tested
+* The `grunt` build passes (executes tests + linting)
+* The work is combined into a single commit
+* The commit messages adhere to our [guideline](https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y)
+
+
+We'd be glad to assist you if you do not get these things right in the first place.