Update readme.md

data processing/readme.md

@@ -45,6 +45,11 @@ PO4 | µmol/kg| phospate concentration, calibration solution was too low, so som
 SiO2 | µmol/kg | silicate concentration	 |
 
 ## Underway data
+The underway data are stored in a matlab file called uw_data_pos519.mat. The data are grouped into structures but they are all on the same time grid.
+The following tabel gives an overview. It will grow with more data being added. The data for each structure are described below.
+1st level|2nd level | unit | description  
+:--- | :--- | :--- | :---
+mmtime | | | Matlab time
 
 ### import dship data