Added HW assembly pictures

HARDWARE_ASSEMBLY.md

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+Some good practices for assembling your node's hardware can be found below.
+
+## Table of Contents
+
+  - [Setup with Metal Case](#setup-with-metal-case)
+  - [Setup with Heatsink Case](#setup-with-heatsink-case)
+
+
+## Setup with Metal Case
+
+1. Apply thermal pads.
+2. Close the case.
+3. Make sure you use the right pins when mounting the display.
+4. Connect the SSD with the raspberry pi.
+5. Plug your device into power and give it internet access to check if
+everything is working as expected.
+
+![HeatsinkCaseAssembly](pictures/heatsink_case/heatsink_case_assembly.jpg)
+
+
+## Setup with Heatsink Case
+
+1. Put screw mounts on the SSD shield.
+2. Prepare screws for mounting the Raspberry PI and corresponding heat sink.
+3. Apply thermal pads.
+4. Attach the heat sink and raspberry pi.
+5. Attach the foot that will later hold things in place.
+6. Make sure you use the right pins when mounting the display.
+7. Fasten the SSD so that it doesn't shake.
+
+![MetalCasePart1](pictures/metal_case/part1_with_numbers.jpg)
+
+
+8. Open the SD card slot.
+9. Attach everything to the case.
+10. Close the case.
+11. Connect the SSD with the raspberry pi.
+12. Insert the flashed SD card.
+13. Remember to use a tamper evident bag before entrusting a third party
+with physical access to the device.
+
+![MetalCasePart2](pictures/metal_case/part_2_with_numbers.jpg)
+
+