Add issue template for GitHub

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,18 @@
+Before filing your issue, please note that it might be more appropriate for Stack Overflow:
+https://stackoverflow.com/questions/tagged/niftynet
+
+Read the following to decide whether to file and issue here or ask a question on [Stack Overflow](https://stackoverflow.com/questions/tagged/niftynet):
+
+¦ Issue                                    ¦ Action ¦
+¦ ---------------------------------------- ¦ ------ ¦
+¦ How do I ...?                            ¦ [Stack Overflow](https://stackoverflow.com/questions/tagged/niftynet) ¦
+¦ I got this error, why?                   ¦ [Stack Overflow](https://stackoverflow.com/questions/tagged/niftynet) ¦
+¦ I got this error and I'm sure it's a bug ¦ Delete this template and file an issue here ¦
+¦ I have an idea/request                   ¦ Delete this template and file an issue here ¦
+
+If you post your question on [Stack Overflow](https://stackoverflow.com/questions/tagged/niftynet), please explain:
+
+1. What you were trying to do (and why)
+2. What happened (include command output)
+3. What you expected to happen
+4. Step-by-step reproduction instructions