The `description` field is mandatory since Guix upstream commit
3948ac25b1dccc40c7fdf56377f94a0775a835ee.
Fixes https://gitlab.com/nonguix/nonguix/-/issues/182
* nongnu/services/vpn.scm (zerotier-one-service-type): Add `description`
field.
Closes#26.
* README.org: Replace `load-broadcom-sta-service` example with new
`kernel-loadable-modules` field.
* nongnu/services/kernel-modules.scm (load-broadcom-sta-service): Update
docstring to recommend switching to `kernel-loadable-modules`.