Update README: login is no longer supported

[uilianries]

Hi!

The section method reference contains login as ConanMultiPackager method, however it was moved to AuthManager.

We have two options, or re-add the login method again, or describe how to login using AuthManager.

/cc @spiderkeys

Regards!

[uilianries]

@lasote Do you think we should introduce the login method again?

I think yes, it's better for users, where it does not require internal elements from CPT, like accessing self.auth_manager from ConanMultiPackager

[lasote]

If someone requests it, yes, it can be propagated to the main class.

[uilianries]

Yes, @spiderkeys wishes the old login method. He needs to login on private remotes to be able to download, it sounds legitimate.

[lasote]

ok