Create pull_request_template.md

Issue: #34

.github/pull_request_template.md

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+Hello, and welcome! Thanks you for your contribution to **Gulliver**.
+
+1. Please let us know about the contribution you are making
+   - [ ] Does this contribution update documentation?
+   - [ ] Does this contribution update code?
+      - [ ] Does it fix a bug?
+      - [ ] Does it add new functionality?      
+
+1. Assuming there is a code change in this contribution
+   - [ ] Have you included unit test(s) to verify the change?
+   - [ ] Do all pre-existing unit tests pass as expected?
+   - [ ] Does the change break existing functionality, if so have you explained why doing so is necessary?
+   - [ ] Have you listed the possible side-effects or negative impacts of the code change.
+
+1. Put `closes #XXXX` in your comment, where XXXX is the the issue that your PR fixes.
+1. Provide any other information that pertinent to the PR you're making. 
+1. Click "Create pull request".
+
+Thanks again, we'll review your PR and and act accordingly.