Add license information

[cornelius-behrend]

Hey there, thanks for adding this plugin.

We've tested it with iOS where we discovered some issues as well as some missing features (i.e. support for allowedCredentials). Would it be possible for you to add a license information to this repo so that we can share the changes in a fork? Maybe they are valuable to others as well. We would prefer the usage of one of these licenses: Apache 2.0, BSD Style License, MIT, ISC.

Thanks in advance, Cornelius