Update instructions for rendering locally

[tsalo]

Here are my notes from the docathon:

brew install [email protected]
brew install chruby

In my ~/.bash_profile I added:

export PATH="/usr/local/opt/[email protected]/bin:$PATH"
source /usr/local/opt/chruby/share/chruby/chruby.sh
RUBIES+=(/usr/local/opt/[email protected])

alias bejs="chruby [email protected];bundle exec jekyll serve"

Then in the terminal again:

cd /path/to/repo
gem install bundler
bundle install

Now I just do bejs to start building the site.
Probably needed a bundle install in the repo's folder at some point too.
Basically, OSX is weird about Ruby. I'm pretty sure it doesn't let you modify your builtin Ruby install, so I used homebrew to install a newer version and then chruby (https://github.com/postmodern/chruby) to switch between Ruby versions. bundler is a tool that will install all of the requirements for a given Ruby "package" (i.e., the website, based on its Gemfile)