Add PR template in advance of Continous Deployment

.github/pull_request_template.md

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+:warning: This application is Continuously Deployed: :warning:
+
+- Merged changes are automatically deployed to staging and production.
+
+- Make sure you follow [the guidance for deployments](https://docs.publishing.service.gov.uk/manual/development-pipeline.html#merge-your-own-pull-request) **before** you merge.
+
+- Check your branch is being deployed in the [Release app](https://release.publishing.service.gov.uk/applications/government-frontend), after merging.