Created by: wanghaoshuang

• Add README.md

• Update url of images.

• Add images.

• Add usage.md

• Add demo.md

• Fix demo.md

• Add model_zoo.md

• Fix pretrain models download in model zoo.

• Refine documents.

1. Rename python to Python
2. Fix README.md: vgg->VGG, resnet->ResNet, mobilenet->MobileNet
3. Rename flops to FLOPS

• Move docs of PaddleSlim from models.fluid.PaddleSlim to models.PaddleSlim

• 1. Introduce model format in README.md

2. Merge results of combined strategies into one table.

• Refine README.md

• Refine README

1. Fix some terms in README.md
2. Fix description of model format in README.md
3. Add Channel-Wise into feature list
4. Rename 'paddle mobile' to 'Paddle Mobile'
5. Add comments for dynamic quantization and static quantization
6. Add entry url for Model Zoo in README.md

• Refine README.