add issue template

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+If you have an issue with cutting a particular file, or an unexpected bug or crash, please open developer tools after starting LosslessCut and before doing the failing operation. Then share the output from the javascript console.
+
+If you have an issue cutting a particular file, please also share the file, if possible. Also please try to cut it with the ffmpeg command line directly.
+
+How to open developer tools:
+- Windows: <kbd>F12</kbd> or <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>I</kbd>
+- Mac: <kbd>Cmd</kbd>+<kbd>Opt</kbd>+<kbd>I</kbd>