[Dune] Dune licensing

[Carsten Gräser]

Dear Dune community,

since I stumbles about this issue a few times recently
this is a friendly reminder of the licensing of dune.
While most of dune is licensed under a liberal license
and thus "free software", this is does not mean "free
as in beer". When "using" dune modules, this is general
not problematic. But it does in particular mean:

When you copy and redistribute code from another dune module
(core, staging, or any other) please check if this is permitted
by the license. Normally the answer is "yes". But it often
does requires to redistribute your copy or derived version
under the same license and keep the original copyright notice.

If you want to distribute a copy or modified version under
another license or if the original version does not provide
licensing information at all: Just contact the original authors
and they would normally be happy to help you finding a solution
- e.g. by telling you which license to use, agreeing to
re-license the code, or maybe by simply donating it.

Best regards
Carsten