[low-pri][doc] Add document to explain the design differences between libcst and ast

[jimmylai]

ast and libcst looks similar but different in many design details. To make it more clear and prevent confusion for users familiar with ast (I'm one of them), it worth documents the differences with some paragraphs.

Item with Difference	LibCST	ast
string value (e.g. asname in Import/ImportFrom)	Name	str
Name	value	id
FunctionDef	FunctionDef	FunctionDef and AsyncFunctionDef
decorators in ClassDef	decorators	decorator_list
access statement inside for or while loop	For.body.body[0].body.value (For.body is an IndentBlock, For.body.body[0] is a SimpleStatementLine, For.body.body[0].body[0] is a Expr.)	For.body[0].value (body[0] is an Expr)
Call.args	Sequence[Arg], Arg.value stores the value	Sequence[value]
keyword arg in Call	Arg in Call.args with Arg.keyword is not None	keyword.arg with keyword in Call.keywords
ImportFrom.module	Name or Attribute	str or Attribute