Add explanation of why default methods cannot be used in JDK 8 javax.lang.model

Various types in javax.lang.model contain warnings describing how methods can be added to interfaces in this API, which is atypical for APIs included in the JDK. While default methods are being added as part of Project Lambda, they cannot be used in javax.lang.model for this release since the version of javax.lang.model bundled in JDK needs to also be runnable on top of JDK 7.