Add some related projects to readme

May be add some (not all) related projects to readme? Or describe, why and where derivablejs is better of them:

atom-like:

[js-atom](https://github.com/cjohansen/js-atom),
[$mol_atom](https://github.com/nin-jin/mol),
[$jin_atom](https://github.com/nin-jin/pms-jin/)

mobx-like:

[mobx](https://github.com/mobxjs/mobx),
[cellx](https://github.com/Riim/cellx),
[matreshka](https://matreshka.io/)

rxjs-like streams:

[kefir](https://rpominov.github.io/kefir/),
[most](https://github.com/cujojs/most),
[rxjs](https://github.com/Reactive-Extensions/RxJS),
[es-observable](https://github.com/stefanpenner/es-observable)