add data-title documentation to readme

Author: awerlang

README.md

@@ -79,6 +79,7 @@ All this work is based on the following assumptions:
    * td: responsive-omit-title: title should be ommited
    * td: responsive-omit-if-empty: no row for empty cells
    * td: responsive-dynamic: add it when there's an `ng-if` directive applied to the element
+   * td: data-title: use to override the header for a given row/cell 
 
 ## Installation
 
@@ -99,6 +100,16 @@ All this work is based on the following assumptions:
 
 ## Special cases
 
+### Header doesn't appear for a row / need to override header
+
+It's possible to override a header with a `data-title` attribute:
+
+    <tr>
+        <td data-title="column 1">tom</td>
+        <td data-title="column 2">jerry</td>
+    </tr> 
+
+
 ### Column can be shown/hidden with ng-if
 
 Also, more than one `td` exist for a single `th`...to deal with this add a `responsive-dynamic` attribute: