mechanism for wintun updates

[Jason A. Donenfeld]

Hi Clint,

The cgit has an atom feed https://git.zx2c4.com/wintun/ and you can of
course enumerate tags using the http dumb protocol:
https://git.zx2c4.com/wintun/info/refs . The github mirror also has
its own notification stuff.

I could start sending [ANNOUNCE] emails for minor wintun updates, if
you really think that's necessary, but so far, it seems like it's
mostly a smaller group of developers using it.

Jason