Started trying to document symmetric encryption

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+Primitives
+==========
+
+.. toctree::
+    :maxdepth: 1
+
+    symmetric-encryption
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
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+Symmetric Encryption
+====================
+
+Symmetric encryption is a way to encrypt (hide the plaintext value) material
+where the encrypter and decrypter both use the same key.
+
+Block ciphers
+-------------
+
+Block ciphers work by encrypting content in chunks, often 64- or 128-bits. They
+combine an underlying algorithm (such as AES), with a mode (such as CBC, CTR,
+or GCM). A simple example of encrypting content with AES is:
+
+.. code-block:: pycon
+
+    >>> from cryptography.primitives import BlockCipher, CBC
+    >>> from cryptography.primitives.aes import AES
+    >>> cipher = BlockCipher(AES(key), CBC(iv))
+    >>> cipher.encrypt("my secret message") + cipher.finalize()
+    # The ciphertext
+    [...]
+