Add GitHub issue template

Author: bgilbert

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,11 @@
+## Context
+
+**Issue type** (bug report or feature request): 
+**Operating system** (e.g. Fedora 24, Mac OS 10.11, Windows 10): 
+**Platform** (e.g. 64-bit x86, 32-bit ARM): 
+**OpenSlide Python version** (`openslide.__version__`): 
+**OpenSlide version** (`openslide.__library_version__`): 
+**Slide format** (e.g. SVS, NDPI, MRXS): 
+
+## Details
+