[Rd] license

[Kasper Daniel Hansen]

R-devel now gives a warning for a non-standard license (this may have
happened for a while).

In Rgraphviz we include the Graphviz source code, which is under
Eclipse.  But the rest of the R package is under Artistic-2.0 or at
least contains code from past contributors which were licensed under
Artistic-2.0.

The standard licenses does not really give an option for this
situation, apart from using 'file LICENSE' (with details on this).

However, this situation - including an external piece of software -
seems to me to be increasingly common.  It may be worthwhile to have
the possibility of specifying this in a more machine readable way.
But it may also be too complicated.  I just wanted to bring attention
to the issue.

Kasper