Update readme.md

data/readme.md

-The measurement dataset is split into two parts --- the rural and the urban subset of the drive-test campaign. 
+The measurement dataset is split into two parts: A *rural* and an *urban* subset of the drive-test campaign. 
 
 In each folder, the measurements are bundled on a per-cell basis. Correspondingly, we have a distinct .csv file for each cell.