[DOCS][Dev] Add Security

docs/developer/src/security.md

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+# Security
+
+## Validate vs Sanitize
+You're probably already familiar with the old saying "Never trust your users
+input", if not, you're now.
+
+Sadly, that often worries developers so much that they will _sanitize_ every
+single user input before storing it. That's, to our eyes, a bad practice.
+You shouldn't trust your users, but that should never lead you to break [data integrity](https://en.wikipedia.org/wiki/Data_integrity).
+
+Instead of sanitize before store, you should _validate_ if the input makes sense,
+and tell your client if it isn't.
+
+## Sanitize before spitting out
+
+If a user inputs a string containing HTML tags, you shouldn't strip them out
+before storing. Depending on the context, you should sanitize it before
+outputting. For that you can call `App\Core\Security::sanitize(string: $html)`,
+optionally you can send a second argument specifying tags to maintain `array: ['tag']`.
+
+## Generating a readable confirmation code
+TODO