sandbox: Add a memory map to the sandbox README

We have a few things in the memory map now, so add documentation for this
to avoid confusion. Also note that it is possible to run all tests now.
Signed-off-by: NSimon Glass <sjg@chromium.org>

board/sandbox/README.sandbox

@@ -422,7 +422,19 @@ coverage in U-Boot is limited, as we need to work to improve it.
 Note that many of these tests are implemented as commands which you can
 run natively on your board if desired (and enabled).
 
-It would be useful to have a central script to run all of these.
+To run all tests use "make check".
+
+
+Memory Map
+----------
+
+Sandbox has its own emulated memory starting at 0. Here are some of the things
+that are mapped into that memory:
+
+      0   CONFIG_SYS_FDT_LOAD_ADDR   Device tree
+   e000   CONFIG_BLOBLIST_ADDR       Blob list
+  10000   CONFIG_MALLOC_F_ADDR       Early memory allocation
+
 
 --
 Simon Glass <sjg@chromium.org>