Adding note in Development Internals

[antoniotoineto]

Proposed changes:

• Adding a note to a especific command at Development Internals in README.md, to avoid misunderstandings while creating the virtual environment

Status:

• [ ] added some tests for the functionality
• [x] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)