🧑‍⚖️ ©️ legal: Clarify copyright for DPG certification

This commit adds a new note to the README file about copyright ownership
as documented by the GPLv3 in its "How to Apply These Terms to Your New
Programs" section:

https://github.com/cboard-org/cboard/blob/e899dcac56b456794f5f9407d202513fd68b9433/LICENSE#L623

Of note, it clarifies that the intellectual property is stewarded by the
OTTAA Project, following the recent partnership of the two projects:

* DPGAlliance/publicgoods-candidates#559 (review)
* https://twitter.com/UNICEFinnovate/status/1429787336368431104
* https://unicef-if.discourse.group/t/meet-ottaa-project-tech-for-speech-impairment/132?u=jwf

This clarifies feedback that came up in review of Cboard's application
to become a Digital Public Good, and will enable the certification
process to move forward. 🚀

See discussion in DPGAlliance/publicgoods-candidates#559 for more context.

CC: @martinbedouret @hectoritr @nathanbaleeta

Signed-off-by: Justin W. Flory (he/him) [UNICEF Innovation] <jflory@unicef.org>

README.md

@@ -136,10 +136,13 @@ Support this project by becoming a sponsor. Your logo will show up here with a l
 <a href="https://opencollective.com/cboard/sponsor/8/website" target="_blank"><img src="https://opencollective.com/cboard/sponsor/8/avatar.svg"></a>
 <a href="https://opencollective.com/cboard/sponsor/9/website" target="_blank"><img src="https://opencollective.com/cboard/sponsor/9/avatar.svg"></a>
 
-## Licenses
 
-Code - [GPLv3](https://github.com/shayc/cboard/blob/master/LICENSE)
+## :memo: Legal & licenses
 
-Mulberry Symbols - [CC BY-SA](https://creativecommons.org/licenses/by-sa/2.0/uk/)
+Copyright © 2017-2021, [OTTAA Project](https://ottaa-project.github.io/) & Cboard contributors.
 
-ARASAAC Symbols - [ CC BY-NC-SA](https://creativecommons.org/licenses/by-nc-sa/4.0/)
+This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License version 3 as published by the Free Software Foundation.
+
+* Code - [GPLv3](https://github.com/cboard-org/cboard/blob/master/LICENSE.txt)
+* Mulberry Symbols - [CC BY-SA 2.0 UK](https://creativecommons.org/licenses/by-sa/2.0/uk/)
+* ARASAAC Symbols - [CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/)