update pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Please provide a meaningful description of what your changes will do. Bonus points for including links to related issues, other PRs, or technical references.
+
+A detailed description helps reviewers understand the context of your change and reduces
+the time needed to review.
+
+Please make sure to follow the [contribution guidelines](https://opentelemetry.io/docs/contributing/).
+
+We run checks on all PRs, to ensure that your contribution follows our [style guide](https://opentelemetry.io/docs/contributing/style-guide/).
+
+Many requirements of that guide can be enforced by running `npm run fix:all` locally and committing the changes.
+
+Note, that although those checks need to be green before we can merge your PR, do not
+worry about their status after submitting your PR. We will provide you with guidance, when and how to
+address them.
+
+Similarly, do not worry about your PR being out-of-date with the base branch and do not continuously
+update or rebase your branch. We will let you know when it is necessary.
+
+If you have any questions, feel free to ask.
+-->