Add ISSUE_TEMPLATE.md [skip ci] (#279)

* Add ISSUE_TEMPLATE.md [skip ci]

Author: Limon Monte

ISSUE_TEMPLATE.md

@@ -0,0 +1,27 @@
+### Description
+<!-- Example: `vaadin-upload` doesn't trigger the "Select files" dialog. -->
+
+### Expected outcome
+<!-- Example: The native "Select files" dialog is shown. -->
+
+### Actual outcome
+<!-- Example: Nothing happens. -->
+
+### Live Demo
+<!-- The template, click "Remix This" to edit it: https://glitch.com/edit/#!/knotty-neon -->
+
+### Steps to reproduce
+<!-- Example
+1. Put a `vaadin-upload` element in the page.
+2. Click the "Upload Files..." button.
+-->
+
+### Browsers Affected
+<!-- Check all that apply -->
+- [ ] Chrome
+- [ ] Firefox
+- [ ] Safari
+- [ ] Edge
+- [ ] IE 11
+- [ ] iOS Safari
+- [ ] Android Chrome