add note to readme regarding release flag

readme.md

@@ -15,6 +15,8 @@ git submodule update --init --recursive
 ./waf build --release
 ```
 
+(NB: the `--release` flag creates builds specifically for armv8/cortex-a53 instruction set, meaning optimized for rpi3 and compatible with rpi4. It also enables aggressive compiler optimizations. Omit flag if you need debug symbols or to build for a different architecture. It does need to supplied to both configuration and build steps.)
+
 ## documentation
 - [user docs](https://monome.org/docs/norns)
 - [API docs](https://monome.org/docs/norns/api)