Add license note to indvidual ADR files

[cornelius]

Follow up to #112: We have an overall license for the repositoriy which covers the ADRs. To follow the example of UIPs and to achieve ultimate clarity it would be good to add a license note to the individual ADR files as well.

[castarco]

As I commented here ( #112 (comment) ) , should we have a policy on which licenses are acceptable as well?

I mean, probably we'll continue using MIT license, but if someone else decides to use a different license, how we'll proceed?