Add Docker image usage to README.md

[jnogol]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions
• Remember to set idealista:develop as base branch;

Description of the Change

Noticing in #57 that Docker image usage was not clear enough, I've added a little explanation with a sample command.

Benefits

Docker image usage is clearer.

Possible Drawbacks

None

[a-zen]

Looks good :-)