Fix examples in Readme

[Qix-]

The readme has some outdated ideas and slightly 'off' syntax (e.g. ECHO you are {!win32 NOT }running on Windows) which is now incorrect or (purposefully) won't parse (e.g. in that example you'd use a backtick quoted string to make sure whitespace gets parsed correctly - as it should be).

[Qix-]

Plug there's stuff that's simply not true anymore - e.g. there's no concept of "build contexts" anymore. Tag's vision has changed slightly, and I felt it best to get a bit lower level than all of that.

[Qix-]

Oy, not to mention SET syntax lol. SET foo=bar would have been a pain to parse.

[Qix-]

Also, it's no longer win32 but instead windows. Likewise, macos is really darwin now.