Document how to upgrade SDK version

[lucianojoublanc-da]

It's somewhat unobvious, though simple once you know how. You'll find something like this in github:

1. And have to change the root daml.yaml file in two places: daml-sdk and sdk-version. These need to match the numbers in the screenshot; the SDK/daml version are not the same. The former appears to have the commit hash appended to it.
2. As mentioned in the daml.yaml file , you need to run bin/update-daml-hashes in order to update the nix/ derivations; otherwise nix will fail saying there is a hash mismatch.