nrf/README: Add use of "make submodules" in alternative build paragraph.
Add "make submodules" to commands when building for the first time.
Otherwise, on a first time build, the submodules have not been checked out
and a lot of `fatal error: nrfx.h: No such file or directory` errors are
printed.
@@ -62,6 +62,7 @@ By default, the PCA10040 (nrf52832) is used as compile target. To build and flas
Alternatively the target board could be defined:
+ make submodules
make BOARD=pca10040
make BOARD=pca10040 deploy
