Add post about the `db-query-matchers` gem

Change-Id: If12791a318db5770895ddefc1a82603aa2131208
Reviewed-on: http://gerrit.causes.com/37048
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>
Tested-by: Henric Trotzig <henric.trotzig@causes.com>