Java Script to change a field fom read to write based on a drop down list item selected.

I am trying to write a java script in adobe forms that changes a field from read to write based on the users selection in a drop down list.

E.g. I have a drop down list with 3 payment terms "net 30"; "Net 60" and "Other" if the user selects other I want the field DescribeOther to become writeable where the user can describe the payment terms.  If the user does not select other than the DescribeOther field stays read only.