Add missing step of installing python packages to README.md
Add missing step of installing python packages to README.md

 - also add note on expected build time on regular machine

h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-3