message: Add documentation on how to encrypt with streaming encryption in Python.
diff: @@ -224,6 +224,34 @@ daead_primitive = keyset_handle.primitive(daead.DeterministicAead)
  ciphertext = daead_primitive.encrypt_deterministically(plaintext, associated_data)
  ```
  +### Symmetric key encryption of streaming data
  +
  +You can obtain and use a
  +[Streaming AEAD (Streaming Authenticated Encryption with Associated Data)](PRIMITIVES.md#streaming-authenticated-encryption-with-associated-data)
  +primitive to encrypt or decrypt data streams:
  +
  +```python
  +import tink
  +from tink import streaming_aead
  +
  +long_plaintext = b'your data...'
  +associated_data = b'context'
  +
  +# Register all streaming AEAD primitives
  +streaming_aead.register()
  +
  +# 1. Get a handle to the key material
  +keyset_handle = tink.new_keyset_handle(streaming_aead.streaming_aead_key_templates.AES256_CTR_HMAC_SHA256_4KB)
  +
  +# 2. Get the primitive.
  +streaming_aead_primitive = keyset_handle.primitive(streaming_aead.StreamingAead)
  +
  +# 3. Use the primitive.
  +output_file = open("ciphertext.out", 'wb')
  +with streaming_aead_primitive.new_encrypting_stream(output_file, associated_data) as enc_stream:
  + bytes_written = enc_stream.write(long_plaintext)
  +```
  +
  ### Message Authentication Code
  You can compute or verify a
  