contrib/openrc-init: Add an OpenRC init script

References:
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=2&chap=4#doc_chap4
https://wiki.gentoo.org/wiki/OpenRC
README: Convert from Markdown to reStructuredText for PyPI
README.rst: Add symlink for GitHub rendering
package-cache: Remove .py extension

Users shouldn't care what the implementation language is.
contrib/openrc/init.d/package-cache: Nest more deeply

Gentoo's doinitd can't handle renaming the files it installs, so
create a deeper tree where the init script can be called
'package-cache'.  This layout leaves room for a future
contrib/openrc/conf.d/package-cache if we want to supply one.