typo: spell 'locally' correctly in README.md.

* README.md (Usage): Spell locally correctly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
README: note use of slingshot_files variable.

* README.md (Installation): Note use of slingshot_files variable.

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