Fix readme for actions

[mhkeller]

Under the new Actions section, this sentence isn't quite right:

This will clone the contents of user/another-repo on top of the existing repo.

It should read as something like this:

This will clone the contents of user/another-repo into the current directory. If duplicate files exist, the one in the current directory will overwrite the one in any cloned-down repos. This allows you to easily customize the final state of your scaffold when combining multiple repos.

Last sentence could also be cut if the intent of this behavior is clear enough.

[mhkeller]

Docs for actions should be clarified in #65