Proposal: Add a more prominent note about security

[frispete]

Hi @osteele,

nice module, but may I ask you to add a more prominent note about security in README.rst?
It doesn't need as detailed as in cautions.rst, but users of keyrings should always be aware about the security primitives without digging into the details.

Wouldn't it be fun to combine our approaches?

Cheers,
Pete