Emphacise need to pre-create config.toml

The README did not make it clear enough that a blank `config.toml` file
needs to be pre-created before runner registration.  Otherwise the
`register` *runlabel* will silently fail to bind-mount the file into the
proper location inside the container.

While we're at it, also add a small example of why it may be a good idea
to edit the `config.toml` file before commiting it as a secret.

Signed-off-by: Chris Evich <cevich@redhat.com>

Containerfile

@@ -43,7 +43,7 @@ ARG NESTED_PRIVILEGED=true
 # may be changed if you know what you're doing.
 LABEL register="podman run -it --rm \
  --secret=REGISTRATION_TOKEN,type=env \
- -v ./config.toml:/home/runner/.gitlab-runner//config.toml:Z \
+ -v ./config.toml:/home/runner/.gitlab-runner/config.toml:Z \
  -e REGISTER_NON_INTERACTIVE=true \
  -e CI_SERVER_URL=https://gitlab.com/ \
  -e RUNNER_NAME=pipglr \