Update instructions for installing Julia

[jdebacker]

It'd be great if the virtual environment could automatically install Julia and the necessary packages. [PyJulia] may allow for this, but in the meantime, I think the instructions for installing Julia could be improved:

1. When I install Julia on a Mac from the download link at Julialang.org, I am not able to access that version of Julia through my terminal. I'm sure there are other steps that can make this happen, but those should be documented in this project.

• Using homebrew to install gave me no issues and this installation of Julia was available from the command line.
• Linking to instructions like these could be helpful.

2. Give instructions for installing the necessary Julia packages.
3. Add Tulip to the list of Julia packages that need to be installed.