Adds action for the update command, and instructions. Fixes #41.

[kerchner]

Recommended test:

• Make sure that your loader.docker-compose.yml is pulling from local code, i.e.

    #image: gwul/tweetsets-loader
    build:
        context: ..
        dockerfile: Dockerfile-loader

• Note an existing collection's dataset metadata as viewed in the UI.

• Modify the collection's dataset.json file in its dataset loading folder.

• Run update instruction in this branch's README.

• Refresh the UI, and observe that the dataset metadata is updated with the change you made to dataset.json

[lwrubel]

Looks good to me!