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>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index ebf5eea..d01d38c 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -72,6 +72,7 @@
    cmd/printenv
    cmd/pstore
    cmd/qfw
+   cmd/read
    cmd/reset
    cmd/rng
    cmd/sbi
@@ -92,6 +93,7 @@
    cmd/ut
    cmd/wdt
    cmd/wget
+   cmd/write
    cmd/xxd
 
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