Add Homebrew installation instructions for macOS GUI

[loganrosen]

Now that a cask for mozregression-gui was added, recommend Homebrew as the primary installation method for macOS users, with direct DMG download as an alternative. Include link to brew.sh for users who need to install Homebrew first.

[loganrosen]

@zzzeid Could you please take a look at this documentation improvement when you get the chance?

[zzzeid]

@zzzeid Could you please take a look at this documentation improvement when you get the chance?

Hi Logan, thanks for the contribution.

Given that mozregression has a flexible release schedule (more or less "as needed", sometimes more frequent than other times), I noticed that the cask and SHA are hard-coded to version 7.0.0. What's the behaviour going to be when a new release is deployed? I am hesitant to add instructions to use homebrew if there isn't a way to automatically pickup the latest version (unless I'm misunderstanding what the hard-coded version means). What kind of overhead is there when we deploy a new release?

The other issue potentially here is that the dmg that is being used in the cask requires a user to manually drag the app into the applications folder (or move it elsewhere) and the GUI app is not really optimized to be launched from the command line -- so I am not sure if this will introduce possibly unexpected issues, especially when we change anything in the way we package it, or with upgrades to PyInstaller or otherwise . For the CLI tool generally pip is used to install the latest (or development) package from PyPI and is the recommended way of installing mozregression.

For these reasons and to avoid having to recommend users install external packages before being able to use mozregression, I'm hesitant to recommend using Homebrew as the suggested method to install mozregression-gui. I'm more open to having it as an alternate/secondary method though as a convenience for anyone who already uses Homebrew.