Add documentation about SpecBindings

[jecisc]

Currently, there is only a Morphic binding to Spec so it is not really useful to know how to use another binding.

When we will have a second one we need to be able to use it.

In order to do it, we can use the SpecBindings (available in Pharo 8) dynamic variable.

Example:

SpecBindings value: MorphicAdapterBindings during: [ myUI openWithSpec ].

It needs a documentation somewhere in the book later.