Create Contributing.md

This adds a very basic contributors file.

Author: rgetz

CONTRIBUTING.md

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+# Contributing to the libIIO
+
+When contributing to this repository, please first discuss the change you wish to make via the 
+[issue tracker](https://github.com/analogdevicesinc/libiio/issues) before making a pull request. 
+
+Please note we have a code of conduct, please follow it in all your interactions with the project.
+
+The [libIIO repository](https://github.com/analogdevicesinc/libiio) is a aggregate of a library 
+and separate applications/programs/examples and doc which use that library:
+* the libiio library (which is released and distributed under the LGPL 2.0 or greater license) and 
+* examples and test code (which is released and distributed under the GPL 2.1 or greater license).
+* groff source for man pages are distributed under the GPL 2.1 or greater.
+
+Any pull requests will be covered by one of these licenses.
+
+## Pull Request Checklist
+
+1. Commit message includes a "Signed-off-by: [name] < email >" to the commit message. 
+   This ensures you have the rights to submit your code, by agreeing to the 
+   [Developer Certificate of Origin](https://developercertificate.org/). If you can not agree to
+   the DCO, don't submit a pull request, as we can not accept it.
+2. Commit should be "atomic", ie : should do one thing only. A pull requests should only contain 
+   multiple commits if that is required to fix the bug or implement the feature.
+3. Commits should have good commit messages. Check out [The git Book](https://git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project)
+   for some pointers, and tools to use.
+4. The project must build and run on MacOS, Windows, and Linux. This is checked on every pull request by the 
+   continuous integration system, things that fail to build can not be merged.
+
+## Pull Request Process
+
+1. Make a fork, if you are not sure on how to make a fork, check out [GitHub help](https://help.github.com/en/github/getting-started-with-github/fork-a-repo)
+2. Make a Pull Request, if you are not sure on how to make a pull request, check out [GitHub help](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request-from-a-fork)
+3. Before a Pull Request can be merged, it must be reviewd by at least one reviewer, and tested on as
+   many different IIO devices as possible. If you have tested it, you can indicated that in your commit
+   message.
+