Push should accept credentials or have better doc

Hello folks!

I see there are two ways of using `push`.

1. `:repositories` key is set: I can use `--repo` for selecting the one I with to use.
2. I use `--repo-map`

But what if I need credentials?

If I understood correctly...I could use option 1 but I cannot insert user and psw. I have to use option 2 but then I need to know the format of the map and I had to check `boot.aether` to find out.

So either we add the two options or we add more doc to push, or both :smile: