Document how to provide credentials

[errm]

The README says

configuring another credential helper should allow pushing to a different registry

It would be great to include an example

[carlossg]

dupe of #109

[Boolman]

this works for me:

sudo docker run -v ${PWD}:${PWD} -v ${HOME}/.docker/config.json:/root/.docker/config.json boolman/kaniko:ubuntu -d gitlab.domain.local:4567/org/containername:tagname -c ${PWD}

cat $HOME/.docker/config.json
{
"auths": {
"gitlab.domain.local:4567": {
"auth": "dXNlcm5hbWU6cGFzc3dvcmQ="
},
},
"HttpHeaders": {
"User-Agent": "Docker-Client/18.03.0-ce (linux)"
}
}

[carlossg]

Documented in #129

[dlorenc]

Closing as a dup of #109.