Added sonarcloud badge

[nikcio]

Added a sonar cloud badge to the README to easily link the project sonar cloud page. This also helps convey the rated maintainability of the project.

Looks:

[NightOwl888]

Thanks. This will make a great addition, except I think we should hold off on merging it until after we have:

1. Updated the CONTRIBUTING.md page to inform contributors not to submit PRs for random things out of the scan that we haven't decided whether or not we will fix.
2. Added a PR template indicating that only approved issues from SonarCloud will be accepted (approved being that they have been made into a GitHub issue). The PR templates would also allow us to put other guardrails in place so we don't have to reject PRs for people who were just trying to help out, but submitted something we cannot use in this version of Lucene.NET.