Add examples for --config-from and --config-only to network_create.md

[mdlinville]

We need some examples for --config-from and --config-only options to docker network create. @mavenugo can you come up with some rough examples / information, and I'll work with you on it.

See #49

cc/ @thaJeztah @joaofnfernandes

[joaofnfernandes]

The PR that introduced these options in the Docker API, also updated the man pages to include some descriptions how how to use this feature.
Maybe we can use this as a starting point.

[ahh-docker]

@mavenugo @andrewhsu @thaJeztah
Given the PR related is merged, can we close this? If not, can you let me know so we can figure out where to open the issue? Thanks.

[thaJeztah]

Looks like we haven't copied the information from the man pages to the online docs yet. I can have a look at doing that tomorrow

[wungad]

Status on this one?