adds doc snipped for installing via homebrew

[timoe]

Proposed changes

Cryptr is now a homebrew cask. I added a line of documentation how to install via homebrew.

Types of changes

What types of changes does your code introduce to Cryptr?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• I have added necessary documentation (if appropriate)

Further comments

Homebrew is Homebrew/homebrew-cask#62453