Update the "How to Create and Use Font Icons" tutorial to mention infusion-icons

The How to Create and Use Font Icons tutorial should be updated to include information about how to use infusion-icons repo to generate font-icons. It's probably still useful to retain some of the current information for those that don't want to use our node module for generating the font. 

See: 

• https://github.com/fluid-project/infusion-icons
• https://wiki.fluidproject.org/display/fluid/Standard+workflow+in+maintaining+and+creating+icon+fonts
• https://wiki.fluidproject.org/display/fluid/Infusion+Icons+Visual+Style+Guide