Improve README with OsX instructions OR add link to generated asset

[pburkholder]

Getting this to run is a hairy proposition:

moovweb/gokogiri#95 is still open

and even so:

$ brew upgrade libxml2
Error: libxml2 2.9.4 already installed

$ go get github.com/opencontrol/fedramp-templater
# pkg-config --cflags libxml-2.0
Package libxml-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxml-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libxml-2.0' found
pkg-config: exit status 1
# pkg-config --cflags libxml-2.0 libxml-2.0
Package libxml-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxml-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libxml-2.0' found
Package libxml-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxml-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libxml-2.0' found
pkg-config: exit status 1

Perhaps save the end-user some trouble and add the templatized output to the repository, or a link to the output.

[pburkholder]

Addressed with #14