Update README.md

[aronsky]

Describe the limitation of SSID and password (must only consist of letters and numbers; relevant to at least some devices, such as RM Pro and RM Mini 3)

Context

Proposed change

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New device
• New product id (the device is already supported with a different id)
• New feature (which adds functionality to an existing device)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Documentation

Additional information

• This PR fixes issue: fixes #
• This PR is related to:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• The code has been formatted using Black.
• The code follows the Zen of Python.
• I am creating the Pull Request against the correct branch.
• Documentation added/updated.