Create PULL_REQUEST_TEMPLATE.md

Author: oreoshake

.github/PULL_REQUEST_TEMPLATE.md

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+## Adding a new header
+
+Generally, adding a new header is always OK.
+
+* Is the header supported by any user agent? If so, which?
+* What does it do?
+* What are the valid values for the header?
+* Where does the specification live?
+
+## Adding a new CSP directive
+
+* Is the directive supported by any user agent? If so, which?
+* What does it do?
+* What are the valid values for the directive?