I presume that you only want it to become read-only once it has been populated with something. There are a few ways that you can do this, with each option depending slightly on the version/edition of Sugar you are using and the exact rules that need to be enforced.
However a base level resolution for this is to create a duplicate description field and make one a calculated field to copy the value of the other. You would then set dependencies so that the initial fields hides when the calculated one is populated with something, and use the same principal on the calculated field. This will make it look as if the field is suddenly becoming read-only, where in fact it is simply swapping the fields over.
If you are looking for something more than this let me know.
Ah so you have Advanced Workflows, this opens up a new option where you could use this. It depends on the specifics for how you want this to work, and if you need authorised users to be able overwrite the entered text or if it should be locked out for all Users.
