clarify help message about make install

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7c68b0f..f5f10ab 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -224,10 +224,12 @@
   message("--------------+----------------------------------------------------------------")
   message("Command       |   Description")
   message("--------------+----------------------------------------------------------------")
-  message("make install  | Install to ${CMAKE_INSTALL_PREFIX}")
-  message("              |   To change that: cmake . -DCMAKE_INSTALL_PREFIX=yourpath")
-  message("              |   Header files are installed to ${INCLUDE_INSTALL_DIR}")
-  message("              |   To change that: cmake . -DEIGEN_INCLUDE_INSTALL_DIR=yourpath")
+  message("make install  | Install to ${CMAKE_INSTALL_PREFIX}. To change that:")
+  message("              |     cmake . -DCMAKE_INSTALL_PREFIX=yourpath")
+  message("              |   Eigen headers will then be installed to:")
+  message("              |     ${INCLUDE_INSTALL_DIR}")
+  message("              |   To install Eigen headers to a separate location, do:")
+  message("              |     cmake . -DEIGEN_INCLUDE_INSTALL_DIR=yourpath")
   message("make doc      | Generate the API documentation, requires Doxygen & LaTeX")
   message("make check    | Build and run the unit-tests. Read this page:")
   message("              |   http://eigen.tuxfamily.org/index.php?title=Tests")