Docs on verifying releases

[Josh Elser]

Given the recent spate of licensing blunders and a question by Ed 
Coleman, I made some time earlier this week to write out obligations for 
verifying releases on the website.

http://accumulo.staging.apache.org/verifying_releases.html
Reviews/feedback is welcome as always. Please feel free to 
modify/edit/correct it as you see fit. If I got something wrong, let me 
know and I'll fix it.

I wanted to get some eyes on it before I pushed it to prod.

Thanks all.

- Josh