RxJs as peer dependency

[LoganDupont]

I was wondering if RxJS shouldn't be part of the peerDependencies because we are now using the latest version of this plugin which contains "rxjs": "^7.0.0" starting from eslint-plugin-rxjs v3.3.1. See this commit
bbe0dc5

As our project is still using RxJS v6 and the latest version of your plugin this means we are currently using the wrong version of the eslint-plugin-rxjs. But now we don't get any notification that we are using the wrong version and there is also no compatibility matrix in the docs to see which version we should use with which version of rxjs.