Note requirements explicitly.

docs/api-guide/parsers.md

@@ -69,6 +69,8 @@ Parses `JSON` request content.
 
 Parses `YAML` request content.
 
+Requires the `pyyaml` package to be installed.
+
 **.media_type**: `application/yaml`
 
 ## XMLParser
@@ -79,6 +81,8 @@ Note that the `XML` markup language is typically used as the base language for m
 
 If you are considering using `XML` for your API, you may want to consider implementing a custom renderer and parser for your specific requirements, and using an existing domain-specific media-type, or creating your own custom XML-based media-type.
 
+Requires the `defusedxml` package to be installed.
+
 **.media_type**: `application/xml`
 
 ## FormParser

docs/api-guide/renderers.md

@@ -90,6 +90,8 @@ The javascript callback function must be set by the client including a `callback
 
 Renders the request data into `YAML`. 
 
+Requires the `pyyaml` package to be installed.
+
 **.media_type**: `application/yaml`
 
 **.format**: `'.yaml'`