Add OpenSSF Scorecard badge

Adds an OpenSSF Scorecard badge to the ReadMe. Scorecard results are increasingly used to assess the trustworthiness of open source software, e.g., in Google's https://deps.dev.

Botan currently scores 7/10, and has some smaller things to improve on. The results can be viewed when following the badge's link or on [deps.dev](https://deps.dev/project/github/randombit%2Fbotan).

readme.rst

@@ -43,6 +43,10 @@ If you think you have found a security issue, see the `security page
  :target: https://repology.org/project/botan/versions
  :alt: Packaging status
 
+.. image:: https://api.securityscorecards.dev/projects/github.com/randombit/botan/badge
+ :target: https://securityscorecards.dev/viewer/?uri=github.com/randombit/botan
+ :alt: CII Best Practices statement
+
 .. image:: https://bestpractices.coreinfrastructure.org/projects/531/badge
  :target: https://bestpractices.coreinfrastructure.org/projects/531
  :alt: CII Best Practices statement