as it's already required by meson.
Also use the default unit file path (/usr/lib/systemd/user/) for
packaged versions. /etc/systemd/user is intended for manually installed
services.
arch-meson is just a Meson wrapper with correct arch packaging prefixes set.
Updating the README.md to bring it ready for wider release:
* Removed some points that were more implementation details
* Added Contributions section
* Simplified the usage section
* Expanded initial explanation
* Fixed steam launch command to allow the overlay to still work
Others:
* Mark as 1.0
* Update the license file for 2018
