AVRO-4145: Add Avro security model

[opwvhk]

What is the purpose of the change

Describe when using Avro is secure. This is necessary documentation, given the world-wide shift to extend product liability to "digital elements" (this includes software). Users of the Avro library will want to know how using Avro affects the security of their product.

See AVRO-4145.

Verifying this change

This change only adds documentation. No code has been changed.

Documentation

• Does this pull request introduce a new feature? (~yes~ / no)
• If yes, how is the feature documented? (not applicable / ~docs / JavaDocs / not documented~)