Add documentation template (#1257)

Copied from Interlisp .github repo.

.github/ISSUE_TEMPLATE/documentation.md

@@ -0,0 +1,17 @@
+---
+name: Documentation problem
+about: Problems with this web site?
+title: ''
+labels: ''
+
+---
+
+**Errors happen. Please tell us the URL**
+
+
+**What does it say?**
+
+
+**What should it say?**
+
+**Screen shot**