document migrating from istanbul to nyc configuration

see https://github.com/istanbuljs/nyc/issues/309

Open Questions

• should we support both the nyc and the istanbul configuration key in package.json (it's a bit confusing for folks using libraries like babel-plugin-istanbul that they need to set an nyc stanza).
  • I could see the other argument, whatever, nyc is the name of the actual reporting tool get used to it.
• do we need a tool for migration, or could we get away with documenting this on our website?

CC: @jamestalmage, @gotwarlost curious about your thoughts.