Add README for Pro Micro

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mozc-yunomi/README_promicro.md

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+# Gboard Yunomi version with Pro Micro
+
+This is an instruction to build the Gboard Yunomi version with Pro Micro.
+For the general information about the Gboard Yunomi version, please refer to [the project README](./README.md).
+
+
+### Parts
+
+-   Kailh choc key x60
+-   Diode (1N4148) x60
+-   Pro Micro x1
+
+### PCBs
+
+We need two types of PCBs to build this project.
+
+![pcbs](./images/pcbs_promicro.jpg)
+
+#### Key-PCB
+
+![key-pcb](./images/key-pcb.png)
+
+File location: board/key
+
+Shape: Rectangular
+
+Components to be soldered: Diodes and key switches
+
+Number of PCBs required: 12
+
+#### Base-PCB:
+
+![base-pcb](./images/base-pcb_promicro.png)
+
+File location: board/base-promicro
+
+Shape: Dodecagonal
+
+Components to be soldered: Pro Micro
+
+Number of PCBs required: 1
+
+### Steps to build
+
+#### Step 1
+
+Solder diodes D1 to D5 onto the twelve Key-PCBs at the silk-screened locations.
+
+*IMPORTANT:* Diodes on six of the Key-PCBs should be placed in the opposite
+direction.
+
+In other words, you can solder diodes as indicated by the silk-screen for six of them, and you need to solder diodes in the opposite direction for the rest six.
+
+#### Step 2
+
+Solder key-switches to each Key-PCB. Note that the diodes need to be placed on
+the opposite side of the Key-PCB to the key-switches, as shown by the
+silk-screen.
+
+#### Step 3
+
+Create a cylindrical shape by combining the 12 Key-PCBs.
+
+There are six connection points between each pair of Key-PCBs. Connect adjacent
+pairs of boards by soldering the tin-plated wires (see the picture).
+
+However, only connect J11 and J12 between the first and last Key-PCB, instead of
+all six locations.
+
+![connect](./images/connect.png)
+
+![connect2](./images/connect2.jpg)
+
+![connect3](./images/connect3.jpg)
+
+If you have a 3D printer, you may print and use jig.stl to help with this step.
+
+#### Step 4
+
+Solder a Pro Micro to the Base-PCB.
+
+#### Step 5
+
+Connect Base-PCB to the cylinder made in Step 3.
+
+![connect4](./images/connect4_promicro.jpg)
+
+#### Step 6
+
+Install [firmware](./firmware/firmware.ino) using the Arduino IDE. Note that you need to uncomment the line 2 (`#define PRO_MICRO`) to make it work with the Pro Micro version.
+
+#### Step 7 (Optional)
+
+Print and mount the platform (./platform_promicro.stl).
+
+![platform](./images/platform_promicro.jpg)
+
+#### Step 8
+
+Connect the keyboard to a PC or other device that has Google Japanese
+Input/Gboard installed. Enjoy!