Create ISSUE_TEMPLATE.md (#1721)

Add issue template to make dealing with issues easier by having users supply relevant information upfront.

.github/ISSUE_TEMPLATE.md

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+<!--
+    Before opening a new issue, please search through the existing issues to
+    see if your topic has already been addressed. Note that you may need to
+    remove the "is:open" filter from the search bar to include closed issues.
+
+    Check the appropriate type for your issue below by placing an x between the
+    brackets. 
+
+    Please note that issues which do not fall under any of the below categories
+    will be closed.
+--->
+### Issue type:
+[ ] Feature request <!-- Requesting the implementation of a new feature -->
+[ ] Bug report      <!-- Reporting unexpected or erroneous behavior -->
+[ ] Documentation   <!-- Proposing a modification to the documentation -->
+
+<!--
+    Please describe the environment.
+-->
+### Environment:
+* AdminLTE Version:  <!-- Example: 2.4 -->
+* Operating System:  <!-- Example: OSX/Android -->
+* Browser (Version):  <!-- Example: Chrome (Latest) -->
+
+<!--
+    BUG REPORTS must include:
+        * Steps or URL needed to reproduce the bug/issue
+        * Any relevant error messages (screenshots may also help)
+        * A snippet of your markup (where applicable)
+
+    FEATURE REQUESTS must include:
+        * A detailed description of the proposed functionality
+-->
+### Description: