README: add OpenSSF Best Practices Badge

README: add OpenSSF Best Practices Badge

This effort was started long time ago by Mike Perez.[1] I now completed the remaining steps to achieve the Passing level of the OpenSSF Best Practices Badge.[2]

It should be used as an opportunity to implement best practices in the Ceph community. For example, the Passing level was achieved without meeting the optional compliance with Dynamic Analysis or Security Scanning.

[1] https://www.bestpractices.dev/en/projects/2220
[2] https://www.bestpractices.dev/en/criteria/0

Signed-off-by: Ernesto Puerta <37327689+epuertat@users.noreply.github.com>