update ADD sources

SOURCES.md

@@ -7,6 +7,6 @@ GLIMS and NSIDC (2005, updated 2020): Global Land Ice Measurements from Space gl
 **Greenland Ice Mapping Project (GIMP version 1.1)**  
 Howat, I. 2017. MEaSUREs Greenland Ice Mapping Project (GIMP) Land Ice and Ocean Classification Mask, Version 1. [GimpIceMask_90m_v1.1]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/B8X58MQBFUPA. [2020-11-19].
 
-**SCAR Antarctic Digital Database (ADD v7.3)**  
-Gerrish, L., Fretwell, P., & Cooper, P. (2020). Medium resolution vector polygons of the Antarctic coastline (7.3) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/ed0a7b70-5adc-4c1e-8d8a-0bb5ee659d18  
+**SCAR Antarctic Digital Database (ADD v7.3 and v7.4)**  
+Gerrish, L., Fretwell, P., & Cooper, P. (2021). Medium resolution vector polylines of the Antarctic coastline (7.4) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/824b5350-763e-4933-bb76-09f5d24cb033
 Gerrish, L., Fretwell, P., & Cooper, P. (2020). Medium resolution vector polygons of Antarctic rock outcrop (7.3) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/077e1f04-7068-4327-a4f2-71d863f70064