add bug report template (#99)

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,18 @@
+Only use GitHub issues for bugs and feature requests.
+For general support please use https://vaadin.com/support
+
+
+NOTE: Issues concerning explicitly Vaadin 7, 8 or Platform should go to the corresponding repository.
+
+To make sure we can quickly validate and fix the bug, please provide as much as possible of the following information:
+- Description of the bug
+- Minimal reproducible example
+- Expected behavior
+- Actual behavior
+- Versions:
+    - Vaadin Framework
+    - Flow
+    - MPR
+    - Java JRE / JDK
+    - Operating System
+    - Browser (if applicable)