Added information about optimization variables

BUILD.md

@@ -192,6 +192,11 @@ The following build options can be used when running CMake
 * CLIENT_ONLY // Will package only the Interface
 * SERVER_ONLY // Will package only the Server
 
+### Optimization build options
+
+* VIRCADIA_OPTIMIZE - This variable defaults to 1 if not set and enables compiler optimization flags on Linux and MacOS. Setting it to 0 will result in unoptimized build.
+* VIRCADIA_CPU_ARCHITECTURE - This variable contains architecture specific compiler flags which are used if VIRCADIA_OPTIMIZE is true. If it is not set, it defaults to "-march=native -mtune=native", which helps yield more performance for locally used build, but for packaging it needs to be set to different value for portability, for example "-msse3".
+
 ### Developer Build Options
 
 * USE_GLES