Checkbox core (plainbox) now lives in checkbox-ng project

[Sylvain Pineau]

Dear checkboxers,

We've merged plainbox code into checkbox-ng!

Rationale: over the years, we've come to the fact that having two separated
names to present checkbox introduced a lot of confusion, especially for new
adopters and customers.

It does no change anything in terms of features, current test providers are
still compatible with the new checkbox-ng (bumped to major version 1.0.0).

For developers though it means that new patches have to submitted against
lp:checkbox-ng as lp:plainbox is no longer the active branch/project.

We're also sunsetting http://plainbox.readthedocs.io The official and unique
documentation now lives exclusively at http://checkbox.readthedocs.io

Please reach us in #checkbox (freenode) if you have any questions about 
this
transition.

Best Regards,

Sylvain Pineau