add template for new issues

ISSUE_TEMPLATE.md

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+### Prerequisites
+
+* [ ] Put an X between the brackets on this line if you have done all of the following:
+    * Checked the online documentation: https://mimic.physionet.org/about/mimic/
+    * Checked that your issue isn't already addressed: https://github.com/MIT-LCP/mimic-code/issues?utf8=%E2%9C%93&q=
+    * Checked online common solutions: https://opendata.stackexchange.com/questions/tagged/mimic-iii
+
+### Description
+
+[Description of the issue]
+
+### Steps to Reproduce
+
+1. [First Step]
+2. [Second Step]
+3. [and so on...]
+
+**Expected behavior:** [What you expect to happen]
+
+**Actual behavior:** [What actually happens]
+
+### Additional Information
+
+Any additional information, configuration or data that might be necessary to reproduce the issue.