Mention deletion of the build folder as a strategy for recompiling

Author: gareth-nx

README.md

@@ -163,6 +163,10 @@ cmake --install build
 
 Now you have a working version of stdlib you can use for your project.
 
+If at some point you wish to recompile stdlib with different options, you might
+want to delete the `build` folder. This will ensure that cached variables from
+earlier builds do not affect the new build.
+
 
 ### Build with make