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Create the dimension, right click & select 'done'.
Double click the dimension to open the editor, select the 1st tab then place any text that you want in front or behind the dimension text. If you want to completely replace the dimension with text tick the 'hide dimension text' check box.
One word of caution with nigel's suggestion. Editing the dimensions in this manner is a local override, changing the dimension in the model will not update any text changes you make. If you do not check the "hide dimension text" option changes to the numeric value of the dimension WILL update to the model but any other text changes will not.