Modify README

[tdejager]

The README still lists prefix-dev/rattler-build#334 as a requirement before the CLI can be used instead of the RATTLER_AUTH file. However, as this is merged, I think we can update the readme with the updated CLI?

[garymm]

The readme is out of date, users can't benefit since rattler-build auth login would have to be run:

• after installing rattler-build
• before running rattler-build build
  and the action does both.

So really the README can't substantively change until #4 is done.

[pavelzw]

I should finish my work on #13, then this would all be fixed 😅