Create issue_template.md

Add an issue template

issue_template.md

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+<!--
+#### Video doesn't play?
+
+In most cases, this is because of your browser. The app uses HTML5 video playback. That means it can only play formats supported by your browser:
+
+https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats#Browser_compatibility
+
+Please test this by dragging the video file into an empty tab. There's only a problem with the app if the video is working in the browser, but not in the player. When you open an issue, please supply a video that doesn't work or exact information on the codecs used.
+-->
+
+### Expected behaviour
+Tell us what should happen
+
+### Actual behaviour
+Tell us what happens instead
+
+### Steps to reproduce
+1.
+2.
+3.