typo: use correct spelling of prefix in README.md.

* README.md: s/perfix/prefix/g

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: distribute INSTALL for autotools instructions.

* bootstrap.conf (gnulib_non_module_files): Add doc/INSTALL.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>