Initial Erasure Code Docs

This is just the start of some EC docs, the idea is not to add
more content to this patch but instead just to edit/agree on
what's here so far and get it merged to the feature branch so
we've got some living docs going that are reviewable in smaller
patches (as opposed to how we did Storage Policies).

Change-Id: I3d4ff2a5bbb8cad5d88fba4a116d7a4d3bad8c8c
Implements: blueprint swift-ec
Merge "Initial Erasure Code Docs" into feature/ec