Checkstyle: Category Design (some) + Sizes

Also updated checkstyle.xml to allow for suppression of specific warnings
since we can't do much about the generated static data methods being too long.

Updated README to include steps how to install and configure the
Checkstyle-IDEA plugin to use our checkstyle.xml file.

Change-Id: Iedd809c857ebb6e49e1169ee25adfcfc3a2c9e62