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
• [x] Documentation

Additional information

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

Checklist

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