Create pull_request_template.md

Author: petercorke

.github/pull_request_template.md

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+We thank you in advance for your pull request, and for your interest and support of the Robotics Toolbox for Python.  
+You could help us a lot by:
+
+* Making your pull request relative to the `future` branch.  This is where we fix issues and merge pull requests prior to pushing out a new release.
+* Including a reference/link to any related issues.
+* Providing clear description of the problem you are addressing with the pull request, and the changes proposed .
+