add Apache 2.0 license reference

[andrii-kovalenko-ct]

By reviewing and comparing we selected Apache 2.0 as it expected to be more patent (trolls) protective:

• https://choosealicense.com/
• https://snyk.io/blog/mit-apache-bsd-fairest-of-them-all/
• https://exygy.com/which-license-should-i-use-mit-vs-apache-vs-gpl/