Merge pull request #539 from radiantearth/pull-request-template

Added pull request template

Author: matthewhanson

.github/pull_request_template.md

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+**Related Issue(s):** #
+
+
+**Proposed Changes:**
+
+1. 
+2. 
+
+**PR Checklist:**
+
+- [ ] This PR has **no** breaking changes.
+- [ ] I have added my changes to the [CHANGELOG](https://github.com/radiantearth/stac-spec/blob/dev/CHANGELOG.md) **or** a CHANGELOG entry is not required.
+- [ ] API only: I have run `npm run generate-all` to update the [generated OpenAPI files](https://github.com/radiantearth/stac-spec/blob/dev/api-spec/README.md#openapi-definitions).