Make a note about CMAKE_BUILD_TYPE

Author: chriso

README.md

@@ -28,7 +28,7 @@ Options are:
 - `-DMMAP_PAGES=1`: Allocate pages with `mmap(2)` rather than `malloc(3)`
 - `-DPAGE_SIZE=4096`: Set the page size
 - `-DINLINE_UNSIGNED=1`: Inline unsigned integers between 0 and `INT_MAX`
-- `-DDEBUG=1`: Create a debug build
+- `-DCMAKE_BUILD_TYPE=Release`: Do a release build / enable optimization
 
 ## Usage