Add readme.md file to mixs folder

[only1chunts]

to be done after the repo structure is updated to have the mixs directory containing the generated artifacts see #704

In the mixs directory, add an auto generated readme.md file that contains the definitions of the extensions* to aid users.

*we didn't discuss it at the CIG call (28Nov2023) but the readme should probably include the checklists descriptions too?

It may look something like the document we created when working on the definitions: https://docs.google.com/document/d/1km8P37kM1R9_AWlDnTeWq2HaQ3EA6X0djzBWn9bJL2A/edit