[jdt-core-dev] API changes document

["Philippe P Mulet" <philippe_mulet@xxxxxxxxxx>]

From now on, in prevision of some breaking API changes to occur in the 3.0
stream, we will maintain a document describing these changes, their
motivations and a story to migrate clients relying on obsoleted APIs.

This will ease our documentation effort when closing on 3.0, and provide
insights to ISVs who anticipate moving. This shouldn't refrain us from also
annoting the build notes with a summary of the change as well.

Jerome is investigating some IWorkingCopy API change, and will define the
document with its first entry.