Add an ISSUE_TEMPLATE

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,17 @@
+Link to PDF file (or attach file here):
+
+Configuration:
+- Web browser and its version:
+- Operating system and its version:
+- PDF.js version:
+- Is an extension:
+
+Steps to reproduce the problem:
+1. 
+2.
+
+What is the expected behavior? (add screenshot)
+
+What went wrong? (add screenshot)
+
+Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):