docs(juggler/readme.md): add possible solution ff-build

[SlootSantos]

Fix FF build on MacOS 10.14 (Mojave) by changed xcode-select
installation behavior

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[SlootSantos]

I signed it!

[googlebot]

CLAs look good, thanks!

[aslushnikov]

@SlootSantos awesome, thanks! I'm still on High Sierra, and this makes updating less scary :-)