Merge pull request #1328 from teojgo/doc/add_jsonschema_requirements

[doc] Add 'jsonschema' to documentation requirements

Author: Vasileios Karakasis

docs/requirements.txt

@@ -7,6 +7,7 @@ docutils>=0.14
 idna>=2.6
 imagesize>=0.7.1
 Jinja2>=2.9.6
+jsonschema
 livereload>=2.5.1
 pathtools>=0.1.2
 port-for==0.3.1