Add issue template

Closes #585.

ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+  Please read the contributing guidelines before opening an issue:
+  https://github.com/Thibaut/devdocs/blob/master/CONTRIBUTING.md
+
+  To request a new documentation, or an update of an existing documentation, go here:
+  https://trello.com/b/6BmTulfx/devdocs-documentation
+-->