Add operating warnings and zotino failure

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

src/build_test_firmware.md

@@ -1,5 +1,16 @@
 # Build and test firmware
 
+## Operating hints and warnings
+
+* 😡 **Always** use grounding strap during dis/assembly or other cards' physical operation
+* 😡 **Always** power off devices before un/plugging
+* 🧐 If needed to power-cycle the crate, wait at least 30 seconds before turning them on
+* 🙅 Avoid the boards touching conductive materials - wires, metals. Use at 
+  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
+
 ## Kasli standalone
 
 ### Checklist
@@ -63,7 +74,7 @@ you can use this book's pages, or if there is no instruction for testing your ha
 ### Known issues
 
 * [artiq-zynq#197](https://git.m-labs.hk/M-Labs/artiq-zynq/issues/197) - some cards (Sampler, Mirny, Zotino and others)  
-  do not work properly with some EEM ports. You might need to reconnect the card to the other ports until it gets working.
+  do not work properly with some EEM ports. You might need to connect the card to the other ports until it gets working.
 
 ## Master-satellite setups
 

src/hw/zotino_fastino.md

@@ -48,3 +48,11 @@ Press ENTER when done.
 2. Check that respective pins have voltages as described by `artiq_sinara_test`
 3. If there are [BNC/SMA-IDC adapters](./bnc_sma_idc_adapter.md), also check their voltages - they should be the same
 4. Check LEDs are on
+
+
+## Common problems
+
+### High-freq audible noise and output values all near -0.1 on Zotino v1.4.2
+
+This may happen when power-cycle is too short. Power down the crate, wait at least 30 seconds, and power up again.
+[Issue](https://github.com/sinara-hw/Zotino/issues/37).