Create pull_request_template.md

Author: garyb

.github/pull_request_template.md

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+### Prerequisites
+
+- [ ] Before opening a pull request, please check the DOM standard (https://dom.spec.whatwg.org/). If it doesn't appear in this spec, it may be present in the spec for one of the other `purescript-web` projects. Although MDN is a great resource, it is not a suitable reference for this project.
+
+### Description
+
+<!-- A link to an existing issue, or a summary of the thing to include with a link to the relevant section of the spec -->