Update README.md

[eugenesujing]

Hi, I really appreciate your efforts on this project. It is amazing and I wish more people could have access to it. I just made some editions on README.md to correct some typos and grammar mistakes. Some sections are also revised for better clarity and concision.

Some major changes are explained as follows:

What's slua-unreal?

At the end of this section, an explanation about why we want to use this project is added to emphasize the advantage of this project.

Showcases

A brief introduction about the programs that are currently employing this project is added to show the achievements boosted by this project so far.

Features

Most features are revised to be consistent in format (all nouns).

Performance

A brief conclusion of the testing is added before the table to highlight the difference in performance among three ways of wrapping.

I hope these changes could help more people see the value of this project.

Please let me know if there is any question or issue about my editions.

[tencent-adm]

All committers have signed the CLA.

[pangweiwei]

thank you