make redis update section match nginx update section

Author: coding-horror

docs/INSTALL-ubuntu.md

@@ -51,7 +51,11 @@ Install necessary packages:
     # Run these commands as your normal login (e.g. "michael")
     sudo apt-get -y install build-essential libssl-dev libyaml-dev git libtool libxslt-dev libxml2-dev libpq-dev gawk curl pngcrush
 
-## Install latest stable redis, package in distro may be a bit old
+## Caching: Redis 
+
+Redis is a networked, in memory key-value store cache. Without the Redis caching layer, we'd have to go to the database a lot more often for common information and the site would be slower as a result.
+
+Be sure to install the latest stable Redis, as the package in the distro may be a bit old:
 
     sudo add-apt-repository ppa:rwky/redis
     sudo apt-get update