Polish English writing of Chapter 5

Fix #374

Fix out of order

Polish English writing in Chapter 2

Make serve server use multi-thread handler, to prevent HTTP OPTIONS g…

Update end-to-end MNIST model URL

Do not create new thread for inference

Paddle will initialize thread complexly and create a lot of thread local variables.

add mnist demo client

add description about port of server

* Fixing few typos in the English readme of Ch1 and Ch2
* Running pre-commit for converting markdown to html

Update chapter 03 and Chapter 06

Add serve API notes

Update README.md

config with gpu