Reference to apt+pyserial seems out of place or unneeded.

[altendky]

Introduced in: 62ec0c6

https://github.com/sunspec/pysunspec/blame/9a7cd3a7072c89ef18673cb730ce28ee5d06e2ff/README.md#L72-L76

    on some systems (Debian based), you could be able to download it as a package (raspberry pi for ex.)

    ```
    $ sudo apt-get install python-serial
    ```

If this is really needed (envs are the way to go for development and are described below) then it belongs down in the dependencies section below. It's quite out of place where it is now.