doc: add copy about how to curl SHA256.txt

Currently we include instructions on how to check the sha of a
downloaded tar-ball, but do not include instruction on how to
get the `SHA256.txt` file. This has led to confusion with people
thinking that the SHA256.txt is included in that tarball.

This commit includes instructions on how to use curl to download the
`SHA256.txt` prior to the instructions on how to verify the sha.

Refs: https://github.com/nodejs/help/issues/113
Refs: https://github.com/nodejs/help/issues/137
PR-URL: https://github.com/nodejs/node/pull/6120


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>