m1rc3/usbpwr/: added tests with an unfriendly capacitative load

m1rc3/usbpwr/README2

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+M1pre-rc4 vs. capacitative USB load
+===================================
+
+This is a test of how the current-limiting USB power switch in
+M1pre-rc4 handles a "nasty" capacitative USB load. The worst load
+identified in previous tests with an M1rc3 with altered reset
+logic (but no current-limiter) [1] was an iCon i-Creativ.
+
+[1] http://lists.milkymist.org/pipermail/devel-milkymist.org/2011-December/002488.html
+
+The M1 was set up in the nominal 5.0 V configuration described in
+README, which results in a supply voltage that's about 200 mV
+below what M1 should normally receive:
+
+Source	Input		J21/J3
+-------	---------------	---------------
+5.00 	4.81/4.86/4.90	4.47/4.62/4.72
+
+The worst drop of the 5 V rail that could be observed in about a
+dozen hotplug tries was about 450 mV [2]. Note that the probe used
+in these measurements has a linear error that cause readings to be
+about 2-3% low (~100 mV in this case), i.e., the lowest voltage
+was still about 200 mV above the threshold of the reset chip.
+
+[2] usb-icreativ-50us.png
+
+The tests above were done with a lab power supply that has very
+fast current limiting. A test with a 5 V switching power supply
+similar to the one shipped with M1 showed a reduced drop [3] of
+only about 350 mV.