Document alternative to TrueCrypt

Once the alternative to TrueCrypt has been chosen, document it.

Feature Branch: feature/5373-replace-truecrypt

Parent Task: #5373 (closed)

Related issues

• Blocked by #5932 (closed)
• Blocks #7739 (closed)
• Blocks #7740 (closed)

Original created by @tails on 6052 (Redmine)