Add a new "SECURITY_CONSIDERATION" document to help customers manage External PSKs

[ttjsu-aws]

Problem:

Add a new SECURITY_CONSIDERATION document.
It MUST contain a section on how to safely manage external PSKs, including:

• Use a different PSK for each endpoint, and use a different PSK when acting as a server vs when acting as a client. See the selfie attack.
• If the server name is configured during the ClientHello callback, then so should the PSKs (the PSKs should be set with the server name in mind).
• Do not store your PSKs publicly or send them over plaintext (do we really need to include this?)

Solution:

Adds new documents called SECURITY_CONSIDERATION unders docs/ folder to discuss attack scenarios and ways to prevent them.

• Does this change what S2N sends over the wire? No
• Does this change any public APIs? No. Doc Addition.
• Which versions of TLS will this impact? All.