Add issue template

.github/issue_template.md

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+<!-- This is a template only, remove/add sections below as appropriate (e.g. for a new feature, there might be no 'current behaviour'.) -->
+
+<!-- First indicate whether you want to request an ENHANCEMENT or report a BUG by using the correct Github label in the side panel* -->
+
+**What is the current behaviour?**
+
+<!-- add current behaviour here -->
+
+**If the current behaviour is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug might get fixed faster if we can run your code and it doesn't have extra dependencies. Add a link to a sample repo and/or any relevant code below:**
+
+<!-- add additional links/info here -->
+
+**What is the expected behaviour?**
+
+<!-- add expected behaviour here -->
+
+**Which versions of Matestack, and which browser/OS are affected by this issue? Did this work in previous versions of Matestack?**
+
+<!-- add version and browser(s) affected, where relevant -->