github: add issue template

.github/ISSUE_TEMPLATE.md

+<!--
+If your issue is a usage question, please submit it to the user mailing-list
+users@riot-os.org or to the developer mailing-list devel@riot-os.org.
+If your issue is related to security, please submit it to the security
+mailing-list security@riot-os.org.
+-->
+
+#### Description
+<!--
+Example: Cannot build gnrc_networking application for samr21-xpro board.
+-->
+
+#### Steps to reproduce the issue
+<!--
+Try to describe as precisely as possible here the steps required to reproduce
+the issue. Here you can also describe your hardware configuration, the network
+setup, etc.
+-->
+
+#### Expected results
+<!--
+Example: The gnrc_networking application builds on samr21-xpro.
+-->
+
+#### Actual results
+<!--
+Please paste or specifically describe the actual output.
+-->
+
+#### Versions
+<!--
+Operating system: Mac OSX, Linux, Vagrant VM
+Build environment: GCC, CLang versions (you can run the following command from
+the RIOT base directory: ./dist/tools/ci/print_toolchain_versions.sh).
+-->
+
+<!-- Thanks for contributing! -->