Add farmOS Link

Change the word "farmOS" into a link in the Acknowledgements at the bottom of README.md.  

The URL to use for the link is: https://farmos.org/

The "Links" section of the following document shows how to create a link using Markdown:
- https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax