Add section in intro about Pharo version

[Ducasse]

!!! Pharo version

The current book was written with Pharo 6.0. Pharo 6.0 is still running on most OSes.
Only some exercises depend on Pharo 6.0. We suggest to use it as much as possible.
We are working on a new version for Pharo 8.0.

[Ducasse]

Added this to v1.0

!!! Pharo version

The current book was written with Pharo 6.0 as support. Only some exercises such as TinyChat depends on Pharo 6.0.
We suggest to use it as much as possible. We know that Pharo 6.0 is not working on 64 bits only architecture.
We are working on a new version for Pharo 8.0.