Contrib: better documentation for bootstrap

(cherry picked from commit 10db18e8)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/bootstrap

@@ -204,12 +204,14 @@ Run "make" to start compilation.
 
 Other targets:
  * make install      same as "make"
+ * make prebuilt     fetch and install prebuilt binaries
+ * make list         list packages
  * make fetch        fetch required source tarballs
  * make fetch-all    fetch all source tarballs
  * make distclean    clean everything and undo bootstrap
  * make mostlyclean  clean everything except source tarballs
  * make clean        clean everything
- * make list         list packages
+ * make package      prepare prebuilt packages
 EOF
 
 mkdir -p ../../contrib/tarballs