Added GitHub issue/PR templates; updated CONTRIBUTING policy

.github/ISSUE_TEMPLATE.md

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+<!--
+    Please note: GitHub issues are to be used only for feature requests
+    and bug reports. For installation assistance or general discussion,
+    please join us on the mailing list:
+
+        https://groups.google.com/forum/#!forum/netbox-discuss
+
+    Please indicate "bug report" or "feature request" below. Be sure to
+    search the existing set of issues (both open and closed) to see if
+    a similar issue has already been raised.
+-->
+### Issue type:
+
+<!--
+    If filing a bug, please indicate the version of Python and NetBox
+    you are running. (This is not necessary for feature requests.)
+-->
+**Python version:**
+**NetBox version:**
+
+<!--
+    If filing a bug, please record the exact steps taken to reproduce
+    the bug and any errors messages that are generated.
+
+    If filing a feature request, please precisely describe the data
+    model or workflow you would like to see implemented, and provide a
+    use case.
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+    Thank you for your interest in contributing to NetBox! Please note
+    that our contribution policy requires that a feature request or bug
+    report be opened for approval prior to filing a pull request. This
+    helps avoid wasting time and effort on something that we might not
+    be able to accept.
+
+    Please indicate the relevant feature request or bug report below.
+-->
+### Fixes:
+
+<!--
+    Please include a summary of the proposed changes below.
+-->