Fix the errors in Readme and enhance its clarity (#37)

* update library intro

* update difference

* modify difference

* fix errors & change the order of annoations

- examples of string var should be written as string (e.g., "mode": "light"/"dark")
- fix grammar errors or inappropriate expressions
- unify the expreesion of var type (e.g., "font-size": size --> "font-size": int)
- modify the order of annoations (from changing the graphical appeareance of the mark itself to adding additional elements) and remove "distribution"

* fix errors & check the format

1. string var ("channel": "x"/"y"/"color"/"size"/"theta")
2. check the format
3. reindex the action list (starting from 1 seems more consistent with other sections)