Add PR template (#223)

.github/PULL_REQUEST_TEMPLATE.md

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+Fixes #
+
+## Changes
+
+Please provide a brief description of the changes here.
+
+Note: if the PR is touching an area that is not listed in the [existing areas](https://github.com/open-telemetry/semantic-conventions/blob/main/docs/README.md), or the area does not have sufficient [domain experts coverage](https://github.com/open-telemetry/semantic-conventions/blob/main/.github/CODEOWNERS), the PR might be tagged as [experts needed](https://github.com/open-telemetry/semantic-conventions/labels/experts%20needed) and move slowly until experts are identified.
+
+## Merge requirement checklist
+
+* [ ] [CONTRIBUTING.md](https://github.com/open-telemetry/semantic-conventions/blob/main/CONTRIBUTING.md) guidelines followed.
+* [ ] [CHANGELOG.md](https://github.com/open-telemetry/semantic-conventions/blob/main/CHANGELOG.md) updated for non-trivial changes.
+* [ ] [schema-next.yaml](https://github.com/open-telemetry/semantic-conventions/blob/main/schema-next.yaml) updated with changes to existing conventions.