Firefox Beta: Add 1Password Caveat

Casks/firefox-beta.rb

@@ -15,4 +15,10 @@ cask 'firefox-beta' do
                   '~/Library/Application Support/Firefox',
                   '~/Library/Caches/Firefox',
                  ]
+
+  caveats <<-EOS.undent
+  The Mac App Store version of 1Password won't work with a Homebrew-cask-linked Mozilla Firefox. To bypass this limitation, you need to either:
+    + Move Mozilla Firefox to your /Applications directory (the app itself, not a symlink).
+    + Install 1Password from outside the Mac App Store (licenses should transfer automatically, but you should contact AgileBits about it).
+  EOS
 end