[verawood] Legacy Libraries DEPR: Document V1->V2 export format changes

[kdmccormick]

We'd discussed a couple approaches here:

1. an in-platform option for importing V1 OLX into a V2 lib
2. an off-platform script that merges multiple V1 OLXs into a V2 ZIP
3. good enough docs on the V2 ZIP format which would allow a savvy operator to merge V1 libraries in themselves

I think we were leaning option (3)? <- confirmed
@ormsbee @bradenmacdonald , let's discuss in early March

steps

First, do this: openedx/openedx-core#492

Then, just add a small section explaining how multiple V1 OLXs exports can be merged into a single V2 zip.