Merge pull request #482 from matplotlib/timhoffm-warn-deprecated

Add waning sign in README to emphasize deprecation

Author: timhoffm

README.md

@@ -3,7 +3,7 @@
 Plot on map projections (with coastlines and political boundaries)
 using matplotlib.
 
-**Warning**: this package is being deprecated in favour of
+:warning: **Warning**: this package is being deprecated in favour of
 [cartopy](https://scitools.org.uk/cartopy/docs/latest/).
 
 ## Requirements