Add an issue template for documentation (#613)

.github/ISSUE_TEMPLATE/documentation.md

@@ -0,0 +1,14 @@
+---
+name: "📚 Documentation issue"
+about: Suggest changes to project's documentation
+title: ''
+labels: 'documentation'
+projects: ''
+assignees: ''
+---
+
+**Relevant links*
+Link(s) to the section(s) of documentation that are outdated or otherwise wrong
+
+**Description**
+Description of the changes you would like to see