Create Template for issues (#500)

* Request some information

.github/ISSUE_TEMPLATE.md

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+**This list was created for avoiding duplicated issues, please read this before posting. If yours isn't listed here, delete this and continue.**
+
+- The theme doesn't work in [distro] that doesn't have a GTK+ 3 version higher or equal than 3.14.
+  - Arc does not support version lower than 3.14, sorry.
+- The theme is broken after a GTK update.
+  - Recompile the theme.
+- The theme is broken after installing from a repository.
+  - Check if the repository that you're using is specifically for the distribution (and distribution/GTK version) that you're using.
+- Using XFCE, the notifications looks ugly or are always full.
+  - Please check if Arc is the theme selected on the Notifications configuration.
+- GTK 2 theme looks like Windows 95 and LibreOffice/others looks ugly.
+  - Check if you have the Murrine GTK engine and `gnome-themes-standard` installed.
+- My GTK apps have a transparent background in Ubuntu.
+  - Run this in terminal: `gsettings set com.canonical.desktop.interface scrollbar-mode normal`.
+
+**Mark those:**
+
+- [ ] I read the list above.
+- [ ] I read the README.
+- [ ] I've checked if my issue isn't duplicated.
+
+**And complete this:**
+
+- Distribution:
+- Desktop environment:
+- GTK+ 3 version:
+- GNOME Shell version (if using):