Add license

[rettinghaus]

This PR adds a license as decided during MEI Board meeting.
Also it updates the readme file.

Closes #63

[rettinghaus]

@musicEnfanthen thank you for the review.

[craigsapp]

I will point out that the file https://github.com/music-encoding/sample-encodings/blob/main/MEI_4.0/Music/Complete_examples/Scarlatti_Sonata_in_C_major.mei

is a derivative electronic edition of this file https://github.com/craigsapp/scarlatti-keyboard-sonatas/blob/master/kern/L001K514.krn

which has a more restrictive license:
https://github.com/craigsapp/scarlatti-keyboard-sonatas/blob/master/LICENSE.txt
https://creativecommons.org/licenses/by-nc-sa/4.0

[th-we]

Other files where I sense licensing problems because they are not public domain:

• Ives_TheCage.mei († 1954)
• McFerrin_Don't_worry.mei (* 1950)

There are some arrangements that might not be kosher, either, e.g. the ones by Michel Rondeau, which seem to be CC-BY:

• Bach-J-C_Fughette_Gmaj.mei
• Bach-J-C_Fughette_No2.mei
• Bach_BrandenburgConcert_No.4_I.mei
• Bach_BrandenburgConcert_No.4_II.mei
• Beethoven_Hymn_to_joy.mei
• Berlioz_Symphony_op25.mei
• Czerny_StringQuartet_d.mei
• Gluck_CheFaroSenzaEuridice.mei
• Handel_Arie.mei
• Handel_Messias.mei
• Hopkins_GatherRoundTheChristmasTree.mei
• Hummel_Concerto_for_trumpet.mei
• Kirnberger_FugeInEFlatMajor.mei
• Lully_LaDescenteDeMars.mei
• Praetorius_PuerNobisNascitur.mei
• Schutz_Jubilate_Deo.mei
• Telemann_Concert.mei
• Telemann_Suite.mei

But I'm not sure if the term "arranger" was used technically correct or if he is really acting as an engraver rather than an arranger. Saint-Saens_LeCarnevalDesAnimaux.mei is however a true arrangement by Martin Packham (* 1978, no license specified).