Add CONTRIBUTING.md

CONTRIBUTING.md

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+Contributing
+============
+
+- Fork, then clone the repo:
+    ````
+    git clone [email protected]:your_username/ModbusMaster.git
+    ````
+
+- Create a topic branch from where you want to base your work
+    - This is usually the master branch
+    - Only target release branches if you are certain your fix must be on that branch
+    - To quickly create a topic branch based on master; `git checkout -b fix/master/my_contribution master`. Please avoid working directly on the `master` branch.
+
+- Follow the [style guide](https://github.com/4-20ma/ModbusMaster/blob/master/STYLE.md)
+
+- Test your change
+
+- Make commits of logical/atomic units
+
+- Check for unnecessary whitespace with `git diff --check` before committing
+
+- Push to your fork and [submit a pull request](https://github.com/4-20ma/ModbusMaster/compare/)
+
+- Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)