Update getting_started.md

added an extended explanation for how Linux users can get and use redis
Update getting_started.md

moved the install of redis for linux into a different section
added a section about installing homebrew for macs

Note that I do not have a mac, so I cannot test the instructions for installing homebrew. If someone would like to confirm or update the instructions as needed, that would be great.
heh, well, I tried

Misunderstood context of previous commit
updated the documentation with information on how to install redis for macs
Merge pull request #44 from HotelCalifornia/patch-2

Update getting_started.md