Release Note: Restrict javax.imageio.spi.ServiceRegistry to ImageIO types

Since Java SE 1.4 `javax.imageio.spi.ServiceRegistry` provided a facility roughly equivalent to the
Java SE 1.6 `java.util.ServiceLoader`.
This image i/o facility is now restricted to supporting SPIs defined as part of `javax.imageio`.
Applications which use it for other purposes need to be re-coded to use `ServiceLoader`.