Document security settings (device-lock) quirks

[dachev]

If I understand correctly a keystore can become inaccessible if the user makes changes to device security settings. It may also be impossible to create a keystore if the devise is missing a proper security lock. It would be very helpful to explain this in the README page and maybe even provide some example code to check for those conditions. Let me know I can update the doc and submit a pull request.