GitHub: Add an issue template

GitHub: Add a template for issues submitted, .github/issue_template.md.
I hope people could provide more needed information when reporting
issues after we provide a number of fields to fill and some extra
instructions in the template, to reduce the time wasted on the issues
because of insufficient information.

See: https://help.github.com/articles/creating-an-issue-template-for-your-repository/

.github/issue_template.md

@@ -0,0 +1,21 @@
+<!-- The template below is for reporting bugs. For feature requests and others, feel free to delete irrelevant entries. -->
+
+Platform: <!-- Example: Ubuntu Desktop 17.04 amd64 -->
+
+GPU, drivers, and screen setup: <!-- Example: NVidia GTX 670, nvidia-drivers 381.09, two monitors configured side-by-side with xrandr -->
+
+Compton version: <!-- Example: 0.1~beta2+20150922-1 from the official repository -->
+
+Compton configuration: <!-- Put the configuration on your preferred pastebin service and place the link here, or directly paste it to the bottom of the issue. -->
+
+### Steps of reproduction
+
+<!-- Please describe the quickest way to reliably reproduce the issue, step by step. With unclear descriptions we might not able to reproduce it, thus would not be able to diagnose or fix the issue. -->
+
+### Expected behavior
+
+<!-- Please describe what the expected behavior is in the case. Sometimes people incorrectly perceive normal behaviors as bugs, and in some other cases we do not expect the same thing as you do, so telling us what you are expecting would be helpful. -->
+
+### Current Behavior & Other details
+
+<!-- Please describe what is the result after following the steps of the reproductions above, in what other cases the issue stops occurring, what you have tried to solve the issue, and what guesses you have on the cause of the issue, etc. -->