In the free version there are only two types of roles - Members and Admins - both of which are allowed to change/delete content from cards, so this will not work for you.
The Observer role in the Business Class (paid) version might work for you. Trello help documentation describes the Observer role as:
Observers - An observer is a member with read-only access to a board.
However, if you couple this up with configuring the Commenting Permissions such that Observers are allowed to comment in cards, then you've got yourself a role that is essentially read-only but with the ability to add comments to cards (keep in mind that a comment can be text, link to another card, or attachments)
Members and Observers - Allow admins, normal members, and observers to comment
So in short, an Observer role configured correctly in Business Class gives you a user role that cannot change/delete what you have written but allows them to add text and attachments through comments on the card. They also will not be able to add new cards.