Update 'README.md'

Added extract_contours to readme

README.md

@@ -78,3 +78,11 @@ Example usage:
 ```
 C:\Python27\ArcGIS10.5\python plot_heatmaps.py param-files\survey-2-avoca.yaml
 ```
+
+### 7. Export 0.7 m contour for DSAS trend analysis
+`extract_contours.py` extracts a predefined contour (say 0.7 m AHD) and outputs it in a format suitable for DSAS shoreline trend analysis.
+
+Example usage:
+```
+python extract_contours.py 
+```