Update README.md

I'm proposing that we link to http://docs.joomla.org/Installing_Joomla_locally instead of the current http://docs.joomla.org/Tutorials:Joomla_Local_install (which has no content) to help Joomla users out installing their Joomla locally.
Merge pull request #5275 from sarahwatz/patch-1

Linking to a page with content