Document disabling worker process in INSTALL

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Tell git to ignore .foreman files

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2259 from opf/feature/dev-procfile

Document disabling worker process in INSTALL