message: Add the issue in EncryptThenAuthenticate to list of known issues.
diff: This doc lists known issues in Tink. Please report new issues by opening new
  tickets or emailing the maintainers at `tink-users@googlegroups.com`.
  +## C++
  +
  +* Before 1.4.0, AES-CTR-HMAC-AEAD keys and the
  + [EncryptThenAuthenticate](https://github.com/google/tink/blob/master/cc/subtle/encrypt_then_authenticate.cc)
  + subtle implementation may be vulnerable to chosen-ciphertext attacks. An
  + attacker can generate ciphertexts that bypass the HMAC verification if and
  + only if all of the following conditions are true:
  + - Tink C++ is used on systems where `size_t` is a 32-bit integer. This is
  + usually the case on 32-bit machines.
  + - The attacker can specify long (>= 2^29 bytes ~ 536MB) associated data.
  +
  ## Java
  * Tink supports Java 7 or newer. Please file a ticket if you want to support
  