update license to Apache 2.0

[eirinikos]

Replace MIT license with Apache 2.0 license.
(All CNCF projects should use the Apache License 2.0 - see opentracing/specification#61 for context.)

We'd like to give previous contributors a 7-day period to air any objections.

(If you have any concerns or feedback, please post a comment here by April 12. Thanks!)

CCing @dankohn @bhs

CCing previous contributors: @tschottdorf @bg451 @RaduBerinde @yurishkuro @nimeshksingh @dvrkps @jmacd @chris-ramon @cce

[chris-ramon]

License would work better instead of Licensing, to match the suffix of the actual license name Apache 2.0 License.

[eirinikos]

I agree. I used Licensing, though, because License would cause Licensee to scan the README as if it were a License file in itself. This issue was discovered with several other OT repos (please see opentracing/opentracing-go#181 (comment)).

Another solution is to omit any mention of the license from the README.

[yurishkuro]

Jaeger uses the word License as the header, and GitHub recognizes the type just fine, e.g. https://github.com/jaegertracing/documentation

[eirinikos]

Yes, I'm not sure why Licensee sometimes recognizes the type (when License is in the header) and sometimes it doesn't.

I didn't encounter this problem when I made changes with OT-cpp or OT-py. When I tested those changes locally, Licensee recognized the type just fine (even with License in the README header).

[yurishkuro]

my advice is to ignore Licensee and use the text that what makes most sense. If github doesn't display it correctly afterwards, we can open a ticket with them to have it fixed (since it's working for other repos, and the actual LICENSE file is unambiguous).

[dankohn]

@yurishkuro GitHub just runs vanilla Licensee. That lets @eirinikos test and find the smallest possible changes that will get GitHub to correctly recognize the license.