cartopy.md and 01_Cartopy_Intro edits

Author: ktyle

core/cartopy.md

@@ -16,5 +16,7 @@ produce maps and other geospatial data analyses.
 Cartopy makes use of the powerful PROJ.4, NumPy and Shapely libraries and includes a programmatic interface
 built on top of Matplotlib for the creation of publication quality maps.
 
+You should have a basic familiarity with Matplotlib prior to working through the Cartopy notebooks presented here.
+
 Key features of cartopy are its object-oriented [projection definitions](https://scitools.org.uk/cartopy/docs/latest/crs/projections.html#cartopy-projections),
 and its ability to transform points, lines, vectors, polygons and images between those projections.