Adding contribution templates (#25)

.github/ISSUE_TEMPLATE.md

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+<!--
+General questions should be asked on the mailing list [email protected].
+
+Before submitting an issue, please fill out the following form.
+-->
+
+### System information
+- **OS Platform and Distribution (e.g., Linux Ubuntu 16.04)**:
+- **Modin installed from (source or binary)**:
+- **Modin version**:
+- **Python version**:
+- **Exact command to reproduce**:
+
+<!--
+You can obtain the Modin version with
+
+python -c "import modin; print(modin.__version__)"
+-->
+
+### Describe the problem
+<!-- Describe the problem clearly here. -->
+
+### Source code / logs
+<!-- Include any logs or source code that would be helpful to diagnose the problem. If including tracebacks, please include the full traceback. Large logs and files should be attached. Try to provide a reproducible test case that is the bare minimum necessary to generate the problem. -->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for your contribution!
+-->
+
+## What do these changes do?
+
+<!-- Please give a short brief about these changes. -->
+
+## Related issue number
+
+<!-- Are there any issues opened that will be resolved by merging this change? -->
+
+- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`