Modify Cmake so that it installs the header file

diff --git a/CMakeLists.txt b/CMakeLists.txt
index fed418f..0354c36 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,4 +19,8 @@ include(GNUInstallDirs)
 install(TARGETS pystring
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
 )
+install (FILES pystring.h
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/${PROJECT_NAME}
+    COMPONENT developer
+)