Remove ambiguities in the COPYING file; add CONTRIBUTING file

Remove ambiguities in the COPYING file; add CONTRIBUTING file

COPYING file:
The motivation of this change is to make it easier for new
contributors to conduct a license and patent review, WITHOUT
changing any licenses.
- Remove references to BSD-style licenses as we have more
  common license exceptions and replace with "other license
  stanzas"
- List the most common situations under which code is licensed
  under licenses other than GPLv2 (section "Licensing Exceptions")
- List the most common non-GPLv2 licenses that are in use in
  this repository based on a recent FOSSology scan (section
  "Licensing Exceptions")
- List other license related conventions within the project
  to make it easier to conduct a license review.
- Clarify the incoming license as its omission has confused
  past contributors (section "Contributions")

CONTRIBUTION file:
The motivation of this file is to make it easier for contributors
to find contribution related resources. Add information on existing
license related conventions to avoid unintentional future licensing
issues. Provide templates for copyright headers for the most commonly
used licenses in this repository.

Signed-off-by: Lars Kurth <lars.kurth@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>