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.

ports/nrf/README.md

@@ -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