[eigen] about eigen's documentation's license

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi,

A few months ago Helmut inquired about our documentation's license and
we didn't really know. There was agreement that we wanted a liberal
documentation licensing, and at the same time we wanted to freely copy
and paste Eigen code into the docs (not to mention automatic
generation with Doxygen).

Helmut just wanted to be able to append Eigen documentation at the end
of his textbook without affecting his own textbook's licensing. In
spirit, this is something that the LGPL should allow, but we were very
unsure about the definition of a 'derivative work' in this case.

Since then I've talked with a Mozilla lawyer about the notion of a
derivative work and I've been confirmed that it's very fuzzy and left
to human appreciation.

So I would propose: let's officially put our documentation under the
same LGPL licensing as the rest of Eigen; and let's add a
LICENSING.txt file clarifying our intent wrt derivative works for
documentation. We can even give the append-at-end-of-book example. Do
you agree?

For the record, it was about the MPLv2 specifically that I asked the
Mozilla lawyer, and in the case of the MPL it's a lot more precise
since it's file-based, so if we were using the MPL, and Helmut kept
our documentation in separate files from his own text, then he would
be fine even without us having to add such a LICENSING.txt file to
clarify this. That said, there are tons of obvious loopholes against
such file-based licensing, which the Mozilla lawyer acknowledged, so
even file-based licensing is still very much dependent on human
appreciation.

Cheers,
Benoit