refactor readme.md instructions for Docker

[bbertucc]

readme.md currently doesn't include any info about a docker installation.

Sample info:

Getting Started

1. Install Docker.
2. Clone the repo and cd the repo's folder.
3. Build a Docker Image with docker build -t equalify-scan ..
4. Create a container with docker run -it --name equalify-scan-container equalify-scan.
5. Execute an axe-cli command with docker exec equalify-scan-container [command].