docs: disabling route controller for self-managed networks (#761)

Currently the README appears to provide incomplete instructions when
using self-managed networks with HCCM. Per #758 setting `HCLOUD_NETWORK`
alone isn't enough as HCCM enables the route controller and fails.
Setting `HCLOUD_NETWORK_ROUTES_ENABLED` to false is also required for
the self-managed network scenario.

This PR adds that to the documentation, and also offers a tip to enable
the `load-balancer.hetzner.cloud/use-private-ip` annotation on the K8s
service (at least in lieu of full documentation of supported
annotations).

If there's a valid use case for self-managed network with route
controller enabled, please let me know and I"m happy to refine the PR to
clarify that.

Fixes #758