Fix pull request template (#36)

Author: thomashoneyman

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,7 @@
+**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 of the change**
 
 Clearly and concisely describe the purpose of the pull request. If this PR relates to an existing issue or change proposal, please link to it. Include any other background context that would help reviewers understand the motivation for this PR.