spi_mode: cmds: Add a small README.

diff --git a/sdcard/spi_mode/cmds/README b/sdcard/spi_mode/cmds/README
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+-------------------------------------------------------------------------------
+SD card / SPI-mode / Commands
+-------------------------------------------------------------------------------
+
+This is a set of example captures of individual SD card commands in SPI-mode.
+
+
+Logic analyzer setup
+--------------------
+
+The logic analyzer used was an FX2-based device (at 24MHz):
+
+  Probe       SD card
+  -------------------
+  0           MOSI
+  1           MISO
+  2           CLK
+  3           CS#
+
+
+cmd17.sr
+--------
+
+This file contains a CMD17 (READ_SINGLE_BLOCK) sequence, reading 512 bytes
+from address 0x000f. The first few bytes of the data are "sigrok rocks".