Adding more visible info about generating omnifixture

tests/resources/OmniFixture-dump.README.md

+The  OmniFixture-dump.sql.gz must be generated via a console command. 
+See [Generating OmniFixture dump](https://github.com/piwik/piwik/tree/master/tests#keep-omnifixture-up-to-date) for more information.