Contributing guidelines should guide prospective contributors on how to get started

**Is your feature request related to a problem? Please describe.**
The current [contributing guidelines](https://github.com/Azure/azure-cli/blob/2c579599b38a73079df6d71c33aeb237b14d3c4c/CONTRIBUTING.rst) explain about the code of conduct, CLA form, etc.

This is useful and important, but it would be equally important to guide potential contributors to `azure-cli` on questions, such as

- how to get a development setup up and running
- what is the overall architecture of `azure-cli`
- any further hints on how to contribute features or fixes that are specific to this project (formatters/linters that need to pass / rules on pull requests / ...)

The contributing guidelines are featured very prominently on the "issues" page - this is great, but it means they will likely be the first place a prospective contributor will look for guidance.

I believe some of this information already exists, e.g. [here](https://github.com/Azure/azure-cli#developer-installation) or [here](https://github.com/Azure/azure-cli#developer-setup), and would just need to be linked up properly.

Conversely, the ["Contribute code" section of the README](https://github.com/Azure/azure-cli#contribute-code) looks like a duplicate of the current content of the contributing guidelines and could be replaced by a link to those guidelines.