Update README.md with macos build & install tip

[Quezion]

Usually I'd bundle this into another PR but the MacOS implementation of bringToFront is ugly and might take a few weeks to land.

[Quezion]

Little update on this stale PR:
I'll clean up the Mac implementation of *isFront this week & get it PR'd, either reusing this one or opening a new one.

The README update in this PR should also be amended to list bringToFront in the large table of implemented features, that way we can callout the missing X11 implementation.

[Quezion]

For some reason, using a local JWM version with publicly released bringToFront code (see merged #269) is failing to correctly steal focus in Mac. The code runs but simply fails to steal focus -- calls to isFront afterwards always returns false. Perhaps this is a permissions issue that the local build works around?

I'm not sure yet but I'll need to PR bringToFront once I figure out a fix. In the meantime, I'm going to close this PR & bundle it into #286 to reduce PR churn.