Adding API Password deprecation information

[generalmat82]

Since API passwords are deprecated, I added information about using API encryption keys, instead of API Passwords to the readme file.

What does this implement/fix?

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code quality improvements to existing code or addition of tests
• [x] Other

Related issue or feature (if applicable):

• fixes

Pull request in esphome (if applicable):

• esphome/esphome#<esphome PR number goes here>

Checklist:

• [ ] The code change is tested and works locally.
• [ ] If api.proto was modified, a linked pull request has been made to esphome with the same changes.
• [ ] Tests have been added to verify that the new code works (under tests/ folder).