Update license to AGPL due to TTL license.

The decision to include the TTL library make KiCad a derivative work
under the terms of the AGPL version 3 or later license.

Add both the GPL3 and AGPL3 license text files and add a readme file
about the KiCad licensing.  Contributing to KiCad still falls under
the GPL3+ license.

Fixes lp:1797095

https://bugs.launchpad.net/kicad/+bug/1797095

Author: stambaughw