Add a link to the spec from the main repository

[domenic]

I could only find https://wicg.github.io/unload-beacon/ by URL-hacking.

Usually I add it over here:

Putting it near the top of the README wouldn't hurt either.

[mingyc]

@yoavweiss Could you help? I don't have permission to update the About section.

[yoavweiss]

Done! Also, you should have permissions to do that now :)

[mingyc]

Thanks!