add pr template

.github/pull_request_template.md

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+## Triage
+
+### Type of change
+
+Please select any of the below items that are appropriate.
+
+This pull request:
+
+- [ ] Adds a new carbon estimation model to CO2.js
+- [ ] Adds new functionality to an existing model
+- [ ] Fixes a bug with an existing model implementation
+- [ ] Add other new functionality to CO2.js
+- [ ] Add new data to CO2.js
+- [ ] Improves developer experience for contributors
+- [ ] Adds contributors to CO2.js
+- [ ] Does something not specified above
+
+### Related issue/s
+
+Please list below any issues this pull request is related to.
+
+- #
+
+### Docs changes required
+
+Do any changes made in this pull request require parts of the [CO2.js documentation](https://developers.thegreenwebfoundation.org/co2js/overview/) to be updated?
+
+- [ ] Yes
+- [ ] No
+- [ ] I don't know
+
+If you answered "Yes", please create an corresponding issue in our [Developer Documentation repository](https://github.com/thegreenwebfoundation/developer-docs).
+
+## Describe the changes made in this pull request
+
+_As clearly as possible, describe the changes made in the pull request. You should at least detail "what changes have been made" and "what the results of these changes will be"._