Improve Readme and doc

I really appreciate this package but I find the documentation difficult for newcomers:
- Various discrepancies in the name of the parameters between Readme, methods' signatures and Doc. For example these 3 names are used for the same parameter: `canExecute`, `restriction`, `registration`. I saw that this problem is mentioned in issue [#47 Nullability support](https://github.com/fluttercommunity/rx_command/issues/47#issuecomment-824623306) which cite another different name, `constraint`.
- The Readme seems to be aimed at those who already have some familiarity with this architecture, which is understandable; in the blog https://www.burkharts.net/apps/blog there are articles suitable for newcomers, but they are not up-to-date (for example, reference is made to the Observable of RxDart, which from version 0.23 directly uses the Stream). Not that this is a problem, I just guess that's why these guides weren't promoted more in the Readme.

I am inexperienced and unfamiliar with git but am available to help update the documentation. Personally, the guide that got me aficionado of this package was the blog article https://www.burkharts.net/apps/blog/rxvms-foundations-rxcommand-and-getit/, together with the two articles of the same series preceding it (which is much more complete than "Making Flutter more Reactive" mentioned in the Readme). One idea could be to update this series of articles to the current RxCommand and RxDart Api and make it available in the wiki of this repo.

Comienza con el README del repositorio y compara sus nombres de parámetros con las firmas de los métodos y la documentación; después, revisa el issue #47 y la serie de artículos enlazada sobre RxCommand y RxDart. Aclara el alcance de la documentación y elige un conjunto coherente de nombres; la tarea estará terminada cuando la guía para nuevos colaboradores refleje la API actual y el material propuesto para la wiki esté actualizado.

Escrito por el modelo de indexación a partir del texto del issue.