Doc: Expand code copying in contributing readme to include license compatibility

[jac4e]

License compatibility is a major issue when it comes to code copying.

Ex. Apache 2.0 licensed code can be used in GPLv3 projects, the reverse is incompatibly (not allowed)

Some general rules:

• no proprietary
• no GPL
• yes MIT
• yes BSD
• yes Public domain

References:

• https://www.apache.org/licenses/GPL-compatibility.html
• https://stackoverflow.com/questions/1978511/is-there-a-chart-of-which-oss-license-is-compatible-with-which

Acceptance Criteria:

• Paragraph touching on license compatibility, licensee compatible with Apache 2.0 and with links to resources