Made a note on the apt-get installer's package name.

This fixes issue #59.

README.mdown

@@ -43,7 +43,8 @@ directory.
 git-flow depends on the availability of the command line utility `getopt`,
 which may not be available in your Unix/Linux environment.  Please use your
 favorite package manager to install `getopt`.  For Cygwin, install the
-`util-linux` package to get `getopt`.
+`util-linux` package to get `getopt`.  If you use `apt-get` as your install
+manager, the package name is `opt`.
 
 
 Integration with your shell