cmake: replace README.txt by README.md

Author: theGreatWhiteShark

CMakeLists.txt

@@ -425,7 +425,7 @@ INCLUDE(InstallRequiredSystemLibraries)
 
 SET(CPACK_PACKAGE_DESCRIPTION_SUMMARY "Hydrogen : an advanced drum machine for GNU/Linux")
 SET(CPACK_PACKAGE_VENDOR "Hydrogen Developers")
-SET(CPACK_PACKAGE_DESCRIPTION_FILE "${CMAKE_SOURCE_DIR}/README.txt")
+SET(CPACK_PACKAGE_DESCRIPTION_FILE "${CMAKE_SOURCE_DIR}/README.md")
 SET(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_SOURCE_DIR}/COPYING")
 SET(CPACK_PACKAGE_VERSION_MAJOR "${VERSION_MAJOR}")
 SET(CPACK_PACKAGE_VERSION_MINOR "${VERSION_MINOR}")