Update warnings

Signed-off-by: Egor Savkin <es@m-labs.hk>

src/build_test_firmware.md

@@ -9,7 +9,10 @@
   least plastic ESD bags if you need the cards to be put at the desk or any other surface.
 * 💁 Be gentle to the EEM ports and any other connectors. Support them when plugging, hold when unplugging
 * 🙆 If you need to take the cards out, take them out one-by-one from the end, unplug EEM and cables if you feel high tension
-* 🙆 Use dedicated power supplies for each crate
+* 🙆 Use dedicated power supplies for each crate, preferably given or equivalent to given by us
+* 🙅 Avoid unnecessary inserts and pullouts, especially of MMCX cables
+
+Failure to comply with this voids the warranty.
 
 ## Kasli standalone