Add defensive programming section to security guide

[msecret]

In order for 18F developers to have a practical understanding of what defensive programming is and the threats of untrusted user data, there should be a section in the security guide about it.

It should:

• [ ] Include information on understanding threats posed by untrusted, user-provided data
• [ ] Either include a link to information about output encoding, or written information
• [ ] The guidance should include practical examples and information on how a team can actually continue this practice on a real team, including tools to use.
• [ ] The link should be reviewed by 18F security lead