Adds import method to add an existing document to the repo

[acurrieclark]

Following discussion in #240, I thought I would add an import method to repo for adding an existing document to the repo.

Points for discussion:

1. Should the parameter be an automerge document, or a Unit8Array? I have made it a document for now, as this made most sense to me, and as it allows us to check the validity of the document and have it typed correctly.
2. What should the error message be which is thrown if an invalid document is passed in.

[kid-icarus]

Should the parameter be an automerge document, or a Unit8Array

Torn on this. If you have a document without loading it from a Uint8Array, you've likely fetched it from the repo, and can use repo.clone(). However, that assumption is admittedly weak, and it's not too much of a hassle to A.load() the Uint8Array into a doc.

[pvh]

I agree that on this interface it should be a Uint8Array -- I'd also suggest a matching export.

[kid-icarus]

Added an export method in #256

[acurrieclark]

Updated the signature to take a Uint8Array instead and added the method to the readme