add template for issues

https://github.com/blog/2111-issue-and-pull-request-templates

move to .github folder + prompt for date

.github/ISSUE_TEMPLATE.md

+<b>OS:</b>
+*Override this line with the exact operating system you are running! (e.g. "Win 7 SP1", "OS X 10.8.5", "Ubuntu 15.10" ...)*
+
+<b>Cockatrice version:</b>
+*Put your Cockatrice version number & build date here! You find them inside the app: `Help` --> `About Cockatrice` (e.g. "2d53ce9 (2016-02-18)")*
+
+___
+<br>
+
+
+*Explain your Issue/Request/Suggestion in detail here!*