Update README with specific example for custom order

It was requested that we improve the documentation around adding custom
orderings to the `PropertySortOrder` linter for those who aren't as
familiar with YAML.

Change-Id: I32a2127cdfdc92d1800c3ecfd1493e3ac5176de2
Reviewed-on: http://gerrit.causes.com/40839
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>