instructions on submodules

README.md

@@ -135,6 +135,7 @@ Other than the building/deployment targets, the following might be of note:
 
 * [create a new repo](https://github.com/microsoft/jacdac-module-template/generate) from `jacdac-module-template`;
   let's say the new repo is called `jacdac-acme-corp-modules`
+* update `jacdac-stm32` and `jacdac-c` submodules (eg., with `git submodule update --remote`)
 * copy `targets/_example/` to `targets/acme-corp-buzzer-v1.0/` (replaceing `acme-corp-buzzer-v1.0` with the name of the module or series of modules)
 * edit [targets/acme-corp-buzzer-v1.0/board.h](targets/_example/board.h) to match your module
 * you likely do not need to edit [targets/acme-corp-buzzer-v1.0/config.mk](targets/_example/config.mk),