Created by: zhouxiao-coder

Update English version of the "Basic Usage" document. #911 (closed)

Changes are made according to advice in #620. Since @tianbingsz has previously updated the Chinese version, I also try to make it consistent with that document.

List of changes:

• Correct grammar mistakes;
• Change equations to follow the syntax of "rst";
• Rewrite some sentences according to suggestions in #620 ;

@luotao1