Add issues template

See: 

* isaacs/github#99 (comment)
* https://github.com/blog/2111-issue-and-pull-request-templates

ISSUE_TEMPLATE.md

@@ -0,0 +1,13 @@
+#### Your system information
+
+* Steam client version:
+* SteamOS version: 
+* Opted into Steam client beta?: [Yes/No] 
+* Opted into SteamOS beta?: [Yes/No] 
+* Have you checked for updates in Settings > System?: [Yes/No]
+
+#### Please describe your issue in as much detail as possible:
+
+
+#### Steps for reproducing this issue:
+