Add a warning about the macOS mandatory code signing requirement

[jesec]

> Warning Unable to sign the macOS executable on non-macOS host
  Due to the mandatory code signing requirement, before the
  executable is distributed to end users, it must be signed with
  the "codesign" utility of macOS, otherwise, it will be immediately
  killed by kernel on launch. An ad-hoc signature is sufficient.
  To do that, "codesign --sign - <executable>" on a Mac.

Ref: https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11_0_1-universal-apps-release-notes

See also: golang/go#42684

[robertsLando]

LGTM

[jesec]

LGTM

Great. To merge, I need an approval, though.

[robertsLando]

Sorry dunno why Ctrl+Enter usually approves the changes