Add Rails 6 credentials example to README.md

[arusa]

No description provided.

[printercu]

Thanks for contributing! WDYT about changing example to edit action? I can find how to edit credentials neither in edgeapi nor in edgeguides. I think that most of users will have same problem, lots of them use rails for the first time.

[arusa]

Sure! I just wanted to show my working configuration and how to get it. I think that helps to see that I use credentials and not secrets and how to access credential files. I think t's a good idea to also show the usage of the edit command.