Update README.md for uvx instructions

[tybalex]

I realized that the PYPI package https://pypi.org/project/postgres-mcp/ exists(and is up tp date).
Thus config with uvx actually works although not mentioned in the Readme. example:

"postgres": {
      "command": "uvx",
      "args": [
        "postgres-mcp",
        "--access-mode=unrestricted"
      ],
      "env": {
        "DATABASE_URI": "postgres://user:password@localhost:5432/proxy"
      }
    }

So here is a PR to add it to README.