Update build README
Update build README

This readme was taken from end-to-end.build, which is now obsolete since the
build files are no longer in a separate repo.

postfix/end-to-end,coruus/e2e-g2y-cryptolib,yahoo/end-to-end,yahoo/end-to-end,coruus/e2e-g2y-cryptolib,dconnolly/yahoo-end-to-end,coruus/e2e-g2y-cryptolib,dconnolly/yahoo-end-to-end,coruus/e2e-g2y-cryptolib,postfix/end-to-end,yahoo/end-to-end,yahoo/end-to-end,dconnolly/yahoo-end-to-end,dconnolly/yahoo-end-to-end,postfix/end-to-end,postfix/end-to-end