Missing valid include for lookup of recording

[SePHTaN]

According to https://musicbrainz.org/doc/MusicBrainz_API isrcs is an allowed entitity when doing a lookup of recordings:

Subqueries
/ws/2/recording artists, releases, isrcs, url-rels
But in Musicbrainz.php this entity is missing in the ValidIncludes array for the recording entity.

It would be great if you add it so it can be used for reverse identifying an track and the associated mb_recordingid as the isrc is more often in file tags than mb id's.
I can also fork your repository, patch and do a pull request if that is your preferred way.

Thanks in advance
Stephan