Add sourcelevel badge to README

[mattstratton]

Ebert has been renamed, or acquired, or whatever, by sourcelevel. The badge in the README should be updated.

Here's the code!

[![SourceLevel](https://app.sourcelevel.io/github/devopsdays/devopsdays-cli.svg)](https://app.sourcelevel.io/github/devopsdays/devopsdays-cli)

(Note - don't forget that the update needs to happen in README.e.md as the README file is dynamically created as part of the build/release process)