add github issue template

Author: nschloe

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,9 @@
+Please provide a minimal working example (MWE) that shows what isn't working as
+expected. Needed is
+
+  * the Python code,
+  * the output TeX (Pgfplots), and
+  * the expected TeX (Pgfplots).
+
+Perhaps the [Pgfplots manual](http://mirror.ctan.org/tex-archive/graphics/pgf/contrib/pgfplots/doc/pgfplots.pdf)
+is helpful here.