Consider adding a THIRD-PARTY-NOTICES.TXT file

[tannergooding]

ImageSharp should consider adding a THIRD-PARTY-NOTICES.TXT file to contain notices for 3rd party code that is referenced or ported into ImageSharp.

This would be especially useful when code is copied from CoreFX to provide certain functionality downlevel on full framework.

[JimBobSquarePants]

Agreed. Super important for future license changes also.