[Doc] : Readme.md - Description and CLI commands to be updated.

I think, the `readme.md ` is quite undescriptive in the aim of this repo(evalai-cli). To elaborate further, I would like to mention 2 changes in readme.md file.

**1. Description.**
To a user who is unaware of EvalAI, the` readme` files makes little sense ( though I know, that user can go [here](https://evalai-cli.cloudcv.org) or search about EvalAI). Nonetheless, for a good first impression, `Readme` file should describe how Eval-cli functions with EvalAI. For example to directly upload training codes on the platform in the form of docker images.

**2. CLI commands**
Though CLI commands are available [here](https://evalai-cli.cloudcv.org) , I think it's a good practice to include the commands with a brief explanation in the `readme` file. It would it more informative.