use cases added to the portable packaging requirements page and a few questions

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Hello all,

I've added three use cases to the portable packaging requirements page
(https://www.w3.org/dpub/IG/wiki/Portable_Package_Requirements).
Hopefully they apply as well as I think they do!

I had three other general questions regarding the document:
1. Should there be a definition of “package” or is that expected to be
basically understood by the target audience?

2. How can streamability be a common requirement to caching or portability?

3. Should this document say anything about the other two states (online;
cached)?

- -Heather
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