README.rst: document mpy-cross gotcha

jepler · jepler · commit 3f82fe8da9ed · 2018-03-20T07:53:30.000-05:00
This was biting me, leading to an odd error later on.
diff --git a/ports/atmel-samd/README.rst b/ports/atmel-samd/README.rst
@@ -141,6 +141,17 @@ The compiler can be changed using the ``CROSS_COMPILE`` variable when invoking
 Building
 --------
 
+Before building the firmware for a given board the MicroPython cross-compiler
+must be built; it will be used to pre-compile some of the built-in scripts to
+bytecode.  The cross-compiler is built and run on the host machine, using:
+
+.. code-block:: shell
+
+    make -C mpy-cross
+
+This command should be executed from the root directory of this repository.
+All other commands below should be executed from the ports/atmel-samd/ directory.
+
 To build for the Arduino Zero:
 
 .. code-block:: shell
