Add / Actualize readme files explaining folders source code.

[aleksmelnikov]

Please, could you add/actualize readme files explaining folders source code. It will be great for explore it.

Todo

• [ ] src/cm: add readme
• [ ] src/core: add readme
• [ ] src/dk: add readme
• [ ] src/id: add readme
• [ ] src/mm: actualize readme
• [ ] src/mt: actualize readme
• [ ] src/pd: actualize readme
• [ ] src/qp: actualize readme
• [ ] src/rp: actualize readme
• [ ] src/sd: actualize readme
• [ ] src/sm: actualize readme
• [ ] src/st: actualize readme
• [ ] src/ul: add readme