Add github issue and pr templates

.github/ISSUE_TEMPLATE.md

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+- xarray-ms version:
+- Python version:
+- Operating System:
+
+### Description
+
+Describe what you were trying to accomplish.
+Tell us what happened, what went wrong, and what you expected to happen.
+
+### Reproducer and Error Logs
+
+```
+Paste the command(s) you ran and the output.
+If there was a crash, please include the traceback here.
+```

.github/PULL_REQUEST_TEMPLATE.md

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+Consider opening an
+[enhancement issue](https://github.com/ratt-ru/xarray-ms/issues/new/choose)
+if the change is large or complex.
+
+We would appreciate it if you could add:
+
+- [ ] Test Cases covering your PR.
+- [ ] Documentation.
+- [ ] A Changelog entry in `doc/source/changelog.rst`.
+
+Development setup information is available [here](https://xarray-ms.readthedocs.io/en/latest/install.html#development).