Update README.md

Add uninstall instructions

Author: fxcoudert

README.md

@@ -6,4 +6,13 @@ The goal of this repository is to host GNU Fortran packages for macOS. These are
 
 ----
 
+### How to uninstall?
+
+```
+sudo rm -rf /usr/local/gfortran /usr/local/bin/gfortran
+sudo pkgutil --forget com.gnu.gfortran
+```
+
+----
+
 *If you are interested in how these installers are built, the documentation is [here](build_package.md).*