Add `vpnkit.descr`: a description for the opam package
Add `vpnkit.descr`: a description for the opam package

This will be copied into the package description by the `make opam-pkg`
step.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit