[chore] Add Github issue and PR templates (#2733)

.github/ISSUE_TEMPLATE.md

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+
+*Note*: for support questions, please use one of these channels: [stackoverflow](http://stackoverflow.com/questions/tagged/socket.io) or [slack](https://socketio.slack.com)
+
+### You want to:
+
+* [x] report a *bug*
+* [ ] request a *feature*
+
+### Current behaviour
+
+
+### Steps to reproduce (if the current behaviour is a bug)
+
+
+### Expected behaviour
+
+
+### Setup
+- OS:
+- browser:
+- socket.io version:
+
+### Other information (e.g. stacktraces, related issues, suggestions how to fix)
+
+

.github/PULL_REQUEST_TEMPLATE.md

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+
+### The kind of change this PR does introduce
+
+* [x] a bug fix
+* [ ] a new feature
+* [ ] an update to the documentation
+* [ ] a code change that improves performance
+* [ ] other
+
+### Current behaviour
+
+
+### New behaviour
+
+
+### Other information (e.g. related issues)
+
+