Update installation instructions
Update installation instructions

Mention the need to run `bundler install` to install required gems.
jeangougou/jeangougou.github.io,jeangougou/jeangougou.github.io,Guilda/Guilda.github.io,rabbots/rabbots.github.io,ageitgey/amplify,rabbots/rabbots.github.io,dsdeiz/fr.ench.info,dsdeiz/fr.ench.info,dsdeiz/fr.ench.info,ageitgey/amplify,jeangougou/jeangougou.github.io,hamchapman/pma-communications,Guilda/Guilda.github.io,hamchapman/pma-communications