Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for your pull request. Please make sure this PR is scoped to one change, and that any added or changed code includes tests with greater than 80% code coverage. See http://prebid.org/dev-docs/testing-prebid.html for documentation on testing Prebid.js.
+-->
+
+## Type of change
+<!-- Remove items that don't apply and/or select an item by changing [ ] to [x] -->
+- [ ] Bugfix
+- [ ] Feature
+- [ ] New bidder adapter
+- [ ] Code style update (formatting, local variables)
+- [ ] Refactoring (no functional changes, no api changes)
+- [ ] Build related changes
+- [ ] CI related changes
+- [ ] Other
+
+## Description of change
+<!-- Describe the change proposed in this pull request -->
+
+<!-- For new bidder adapters, please provide the following -->
+- test parameters for validating bids
+```
+{
+  bidder: '<bidder name>',
+  params: {
+    // ...
+  }
+}
+```
+- contact email of the adapter’s maintainer
+- [ ] official adapter submission
+
+
+## Other information
+<!-- References to related PR or issue #s, @mentions of the person or team responsible for reviewing changes, etc. -->