Document how to override the base font-family to use another font

[mikewheaton]

Separating this work item out from #852. There are limitations on the use of Segoe UI, which prevent developers from using Fabric for projects that aren't specifically for Office, or from hosting their own fonts without relying on the CDN.

The request here is to document how to override the base font-family to set a different font stack, which would prevent Fabric from pulling Segoe UI from the CDN as usual.