additions doc

[BebeSparkelSparkel]

Beyond add-a-package-to-the-package-set I don't see the formats to add a package.

Looking for the way to add a remote github repo from a specific branch.

[f-f]

There is this note in the section just above that one:

Note: you can use a "branch", a "tag" or a "commit hash" as a version. Generally it's recommended that you avoid using branches, because if you push new commits to a branch, spago won't pick them up unless you delete the .spago folder.

Would you have any suggestions on how to improve this and make it clearer?

[f-f]

Fixed in #398