add storage group documentation under seal-workers

[LexLuthr]

Fixes #30

The documentation details how storage groups can be used to reduce the data movement by assigning the sealing steps to a fixed group of nodes with access to already processed data.

[johnnymatthews]

This is a great PR, thanks for submitting! I've suggested a few changes, mainly based around formatting and minor grammar issues. I've not commited them yet, since I think it'd be good for you to take a look and understand the changes I've made.

[LexLuthr]

I have added comments on 2 changes. Everything else looks better than before. Thank you for helping me fix this. I think we are good to merge after you review my 2 comments.