Add issues temaplate

ISSUE_TEMPLATE.md

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+**Before adding an issue**
+Please ensure that your issue is not already reported in our [issues list](https://tree.taiga.io/project/taiga/issues?order_by=-created_date).
+If this issue was already reported, remember that you can upvote it to raise its importance.
+
+**Do you want to request a *feature* or report a *bug*?**
+
+**What is the current behavior?**
+
+**If the current behavior is a bug, please provide the steps to reproduce.**
+
+**What is the expected behavior?**
+
+**Is it happening in taiga.io or in your own instance?**
+
+**What browser/version are you using?**
+
+**Are there any console errors *(Ctrl + F12)* in red?**
+
+Thanks for reporting!