Add issue template

.gitlab/issue_templates/issue.md

+# <Title>
+
+[ ] The model is satisfied when it shouldn't (or viceversa)
+[ ] There's an issue with the model checker
+[ ] There's an error with the DOMLR (DSL for requirements) 
+
+## Files
+To investigate your issue, I need:
+- The DOML version you're targeting
+- The PIACERE IDE version you are using
+- A link to DOML model in question
+- The error message (screenshot or copy-n-paste it)
+- Any other information you think it's useful
+
+## Description