doc(README.md): add missing instruction for cask

Author: soonhokong

README.md

@@ -20,7 +20,7 @@ sudo apt-get install mercurial python2.7 texlive-latex-recommended \
 git clone https://github.com/leanprover/tutorial
 cd tutorial
 tar xvfz header/l3kernel.tar.gz -C ~/
-make install-cask # after this, you need to add 
+make install-cask # after this, you need to add the cask binary to your $PATH
 make install-pygments  
 make
 ```