Create issue_template.md

Author: ChristianGruen

.github/issue_template.md

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+Welcome to the BaseX issue tracker.
+
+* Before adding new feature requests and bug reports,
+  please write to our [basex-talk](http://basex.org/open-source/).
+* You may as well find it helpful to browse through our
+  [documentation](http://docs.basex.org/wiki/Main_Page).
+
+Thanks for your help.