Mention README-hacking for git users.
Issue raised by Jean-Christophe Manciot in https://bugs.gnu.org/25377 .
* README: Improve installation paragraph based on coreutils' README.
First, run this command:
git grep -l ' $'|xargs perl -pi -e 's/[ \t]+$//'
Then some minor fix-up to make the two newly-failing tests
pass once again:
* testsuite/mac-mf.sed: Append this, s/ $//,
to eliminate trailing spaces in the actual output.
* testsuite/y-newline.good: Manually remove a trailing
space between two concatenated prompts.
