Update issue templates

Author: andydavies

.github/ISSUE_TEMPLATE/issue-template.md

@@ -0,0 +1,22 @@
+---
+name: Issue template
+about: Tracking HTTP/2 Prioritisation across browsers, servers and CDNs
+
+---
+
+**Please include server / CDN, and browser in issue title**
+
+### Summary
+Add a summary of the issue
+
+### Server / CDN etc.
+Which server / CDN etc does this issue apply to?
+
+### Browser
+What browsers did you test with?
+
+### Link to WebPageTest result
+If you tested  using WebPageTest please add a link to the relevant result(s).
+
+### Link to  upstream bug report
+If you're reported a bug to the relevant vendor, or project please add a link to it