Added some blurb about why we will differentiate on the hash-generating
process:

--

We could use "shasum", which lets us choose the (SHA based) digest. However,
"shasum" is a Perl script and preliminary tests have shown that it's
twice as slow as "sha1sum" (a C binary) from the coreutils package.

We have some actual numbers now: generating the SHA1 checksum
of a 1 MB file, for 100 runs:
 shasum COUNT: 100 TIME: 38 seconds       - perl
openssl COUNT: 100 TIME:  7 seconds       - openssl
sha1sum COUNT: 100 TIME:  1 seconds       - coreutils

As we're going to have different routines for setting/getting EAs for each
operating system anyway, we'll have different routines for checksums as well.

Signed-off-by: Christian Kujau <github@nerdbynature.de>

	modified:   checksum_files.sh