Update README.md

Ok.  I think I caught them all but these are often famous last words.

I renamed Accept-Header stuff to Localization, leaving `Accept-Header` as needed.
I left the link to /V3/Countries up front but if it is too much, it can go.
I added a lot of detail about the Accept-Language format because (1) the V1.1 and, thus, V3 API's won't tell you when you get it wrong and (2) it isn't necessarily clear.