Clarify use of branch() and steps to create a multiverse in README

[mjskay]

Pinging off of #95, I looked back at the README and it could probably do a better job introducing the use of branch() and ultimately of bringing the user along to creating a single multiverse analysis.

There's a paragraph under the "Multiverse Specification" section that appears to have a TODO for addressing the branch() part of the equation already:

To create a multiverse that includes these three possible processing options, we can use the branch() function. The branch() function ### change here ###

and the code chunk after that is (I think) the first introduction of branch(), but it does not explain that it must be used inside a multiverse block or the inside() function.

One fix might be to move up the introduction of inside() / code blocks (which is right after that) and then introduce branch() afterwards. This would perhaps be a more logical order anyway, as the doc is structured as the steps to take a single analysis to a multiverse, which are currently given as:

1. Define the multiverse object
2. Define a branch
3. Move things into inside() or multiverse code blocks

I would suggest instead:

1. Define the multiverse object
2. Move things into inside() or multiverse code blocks
3. Define a branch