Copyright modifications to CONTRIBUTING.md #44

From @frans-ebu suggestions in email regarding copyright:

• New contributions should have the new contributors' copyright included:
  -- Completely new (e.g. Dolby) contributions should thus include a Dolby copyright header and the BSD-3 clause text.
  -- When changes are made to existing (e.g. BBC originated) files, both the original (e.g. BBC) and new (e.g. Dolby) copyright information should be included.

• It would be helpful to make this explicit in CONTRIBUTING.md.
  -- Spell out bmx is under BSD-3 and contributing means the contributor agrees with this.
  -- Note the contributor should add (not remove) copyright lines as relevant.
  -- Point out the contributor must be sure he is allowed to contribute the code (own work or work compatible with BSD-3 already).

• Related: may be useful to also mandate the use of DCOs ("git commit -s")?