DOCS/contribute.md: license clarifications

Basically, both the license of the file and the preferred license of the
project (LGPLv2.1+) counts. I'm doing that so that files with more
liberal licenses don't get infected by LGPL, but allow copy & pasting to
LGPL source files without jumping through lawyer bullshit hoops.

Mention this in Copyright too.

Copyright

@@ -21,6 +21,8 @@ project can relicense the changes to LGPLv2.1+ at a later point without asking
 the contributor. This is a safeguard for making potential relicensing of
 remaining GPL code to LGPLv2.1+ easier.
 
+See DOCS/contribute.md for binding rules wrt. licensing for contributions.
+
 For information about authors and contributors, consult the git log, which
 contains the complete SVN and CVS history as well.
 

DOCS/contribute.md

@@ -17,10 +17,14 @@ Sending patches
 - All new code must be LGPLv2.1+ licensed, or come with the implicit agreement
   that it will be relicensed to LGPLv2.1+ later (see ``Copyright`` in the
   repository root directory).
+  Changes in files with more liberal licenses (such as BSD, MIT, or ISC) are
+  assumed to be dual-licensed under LGPLv2.1+ and the license indicated in the
+  file header.
 - You must be either the exclusive author of the patch, or acknowledge all
   authors involved in the commit message. If you take 3rd party code, authorship
   and copyright must be properly acknowledged. If the license of the code is not
   LGPLv2.1+, this must be mentioned.
+- These license statements are legally binding.
 - Don't use fake names (something that looks like an actual names, and may be
   someone else's name, but is not your legal name). Using a pseudonyms is
   allowed if it can be used to identify or contact you, even if whatever