message: Add description of envelope encryption to wire format.
diff: @@ -122,6 +122,22 @@ IV || ciphertext.
  AES-GCM-HKDF uses a nonce prefix of 7 bytes to get 12 byte IVs for the segment
  encryption. The key derived is the same size as the input key material.
  +#### Envelope Encryption
  +
  +Envelope encryption encrypts the data with a data encryption key `DEK` using
  +Tink's AEAD primitives. In addition the `DEK` is encrypted with an external
  +provider (e.g. GCP) and prepended to the `ciphertext`. The format for envelope
  +encryption is as follows:
  +
  +```
  +DEK length || encrypted DEK || ciphertext
  +```
  +
  +The `DEK length` is 4 bytes, storing the length of the `encrypted DEK` as a
  +32-bit big endian integer. The format of the `encrypted DEK` depends on the
  +external provider which was used for encrypting the `DEK`. The `ciphertext` will
  +have the exact same format as the AEAD primitive corresponding to the `DEK`.
  +
  ### MAC
  Tink follows the corresponding RFCs.
  