Compare to other songbook tools

[jgvictores]

• https://github.com/wojtex/cantionale + https://github.com/wojtex/songbook-generator
• http://www.softpedia.com/get/Multimedia/Audio/Other-AUDIO-Tools/Songbook-Generator.shtml#sgal_0
• Also see https://github.com/adius/awesome-sheet-music

[jgvictores]

Closed via https://github.com/UC3Music/songbook-tools/commit/bb5c8597abbc28aff74b5af68c7823375c75435e " README add Similar and Related section"