Update Labeling UX README to improve application onboarding experience (

#12)

* refine current README content

* finish outline of updated README

* finish specify value and create field parts

* adjust the outline

* finish assign label section

* finish first version of labeling UX usage tutorial

* finish first version of overall updating (without screenshot and gif)

* fill in prerequisites

* adjust some format of README.md

* Correct naming of application

* Correct README typo

* Update README: no need to create Server/data folder

While we now include Server/data folder in repo, user no longer need to create this folder on their own.

* Add GIFs and Snapshots to README.md

* Content and phrasing refinement

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Co-authored-by: Yi-Ting Wei (Pactera) <[email protected]>