README: warn about potential downsides of option #1 on systems where the UID and 42 login differ (i.e. typically one's own machine).

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Example error/issue below.

% brew postinstall --debug cmake
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/cmake.rb
==> Postinstalling cmake
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FromPathLoader): loading /usr/local/opt/cmake/.brew/cmake.rb
Warning: The post-install step did not complete successfully
You can try again using:
  brew postinstall cmake
==> An exception occurred within a child process:
  ArgumentError: user zaphod doesn't exist

/usr/local/Homebrew/Library/Homebrew/sandbox.rb:78:in `home'
/usr/local/Homebrew/Library/Homebrew/sandbox.rb:78:in `allow_write_xcode'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:1128:in `block in post_install'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:45:in `block (3 levels) in safe_fork'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:37:in `fork'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:37:in `block (2 levels) in safe_fork'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:34:in `open'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:34:in `block in safe_fork'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/tmpdir.rb:93:in `mktmpdir'
/usr/local/Homebrew/Library/Homebrew/utils/fork.rb:33:in `safe_fork'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:1121:in `post_install'
/usr/local/Homebrew/Library/Homebrew/cmd/postinstall.rb:30:in `block in postinstall'
/usr/local/Homebrew/Library/Homebrew/cmd/postinstall.rb:27:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/postinstall.rb:27:in `postinstall'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'
%

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