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.
ref: https://github.com/nodejs/help/issues/113 ref: https://github.com/nodejs/help/issues/137