Document the plan to update ETS to use the new traits observation

[rahulporuri]

Traits version 6.1.0 introduced the new observe way to listen to trait changes - instead of the old on_trait_change handlers. It would be nice to document the plan to update the various ETS libraries to use the new traits observation framework.

Note that updating to the new traits observation framework means dropping support for earlier versions of traits i.e. traits < 6.1.0.