update your metadata walkthrough

[evanwill]

@juliastone0729 just noticed this from #37 from last DHSI feedback-->
walkthrough for the common task of updating your metadata from Sheets. So when a person already has a collection set up and running, but they edit their metadata (tweak rows or add new items, etc), and need to update the version in their repository.

I think might be helpful to walkthrough for how to update in both directions:

• Google Sheet to repository (download, rename the same, upload/commit to _data)
• repository (if you edited the csv in your project) to Google Sheet (import, select correct options, etc)