Add README instructions on how to install xml2rfc

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

README.md

@@ -35,7 +35,7 @@ Labels are assigned based on [Sensible Github Labels](https://github.com/Releque
     * links.json - JSON Hyper-Schema's Link Description Object meta-schema
     * hyper-schema-output.json - The recommended output format for JSON Hyper-Schema links
 
-Type "make" at a shell to build the .txt and .html spec files.
+Install "xml2rfc" using "pip" (https://pypi.org/project/xml2rfc/) and type "make" at a shell to build the .txt and .html spec files.
 
 Descriptions of the xml2rfc, I-D documents, and RFC processes: