Mention macOS version requirement

[ryandesign]

I'd like to request that you mention in the README what version of macOS is required. Looks like it's currently 10.13:

CLK/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

Line 6602 in 90f0e93

MACOSX_DEPLOYMENT_TARGET = 10.13;

but I know you're considering increasing that due to #1133. (I recommend you don't increase the minimum OS version since there are plenty of Macs that cannot officially upgrade past 10.13, but that's for you to decide.)

It would also be nice to mention the macOS version requirement in the text of every GitHub release. That way, someone looking through the list of releases will know which ones work on their OS.

If you do ever increase the minimum OS version—and I see you did at one point increase it to 10.14 and then decrease it to 10.13—it would be nice to mention in the README the last version that worked on a particular OS version, so that someone using an older OS that you no longer work with won't have to hunt for the last working version.