crypto: Add Crypto API documentation

Add a Crypto API page which gives an overview of Mbed Crypto, links to
the PSA Crypto API specification, links to the Crypto repo and its
documents, and explains a bit about how Mbed Crypto is integrated with
Mbed OS for boards with secure environments.

Author: Patater

docs/api/security/crypto.md

@@ -0,0 +1,49 @@
+## Crypto
+
+Arm Mbed Crypto is the reference implementation of the cryptography interface
+of the Arm Platform Security Architecture (PSA). This is a preview release of
+Mbed Crypto, provided for evaluation purposes only.
+
+We have adapted and [integrated Mbed Crypto with Mbed
+OS](https://github.com/ARMmbed/mbed-os/blob/master/features/mbedtls/mbedcrypto).
+On PSA platforms that support it, Mbed Crypto is pre-integrated with Mbed OS to
+take advantage of the platform's segmented architecture and isolate
+cryptographic keys and operations from applications.
+
+You can import Mbed Crypto from its standalone
+[release](https://github.com/ARMmbed/mbed-crypto). Mbed Crypto as integrated
+with Mbed OS does not include all test code or scripts used in the development
+of the library. You can find all of these in the standalone release.
+
+<span class="notes">**Note:** Mbed Crypto, like Mbed TLS, needs a secure source
+of random numbers; make sure that your target board has one and that it is
+fully ported to Arm Mbed OS. You can read more about this in our [porting
+guide](../contributing/index.html).</span>
+
+
+### Mbed Crypto examples
+
+[The Mbed Crypto
+example](https://github.com/ARMmbed/mbed-os-example-mbed-crypto) covers some
+basic use of the PSA Crypto API as well as factory entropy injection.
+
+For further information, refer to the readme file in [the example
+repository](https://github.com/ARMmbed/mbed-os-example-mbed-crypto).
+
+
+### Configuring Mbed Crypto features
+
+The Mbed TLS configuration system configures Mbed Crypto. Refer to [Mbed TLS
+documentation for how to configure Mbed TLS and Mbed
+Crypto](TLS.md#configuring-mbed-tls-features).
+
+
+### Other resources
+
+The [Mbed Crypto website](https://github.com/ARMmbed/mbed-crypto) contains [an
+overview of the PSA Crypto
+API](https://github.com/ARMmbed/mbed-crypto/blob/psa-api-1.0-beta/docs/PSA_Crypto_API_Overview.pdf),
+[the PSA Crypto API
+reference](https://github.com/ARMmbed/mbed-crypto/blob/psa-api-1.0-beta/docs/PSA_Crypto_API_Reference.pdf),
+and [other general developer
+documentation](https://github.com/ARMmbed/mbed-crypto/tree/development/docs).