doc: document read/write commands

The read and write commands are, deliberately, implemented in the same
file, so that they stay feature-compatible (e.g. if someone implements
support for "read the full partition, however large that is", that
same syntax should also work for write). In order to ensure the
documentation for both are similarly kept in sync, and to avoid
duplication, document them both in read.rst, and add a stub write.rst
referring to read.rst.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>