Basic readme with install instructions

Author: emanuelen5

README.md

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+# Pico Macro keyboard (CircuitPython)
+
+## Installation
+
+### Install CircuitPython 7+ on the Pico
+* Download it from here: https://circuitpython.org/board/raspberry_pi_pico/
+* Hold the BOOTSEL button while plugging in the Pico.
+* Drop the CircuitPython.uf2 file in the Pico's CIRCUITPYTHON drive.
+
+### Upload the code to the Pico
+* Clone submodules etc.:
+  ```bash
+  $ ./install.sh
+  Cloning external submodules/libs
+  Copying external libs into pico libs folder
+  ```
+* Copy all files from the [pico](./pico) folder to your CircuitPython drive. And voilà!
+
+## Usage
+* Add mappings to config.json for the pins to key combinations
+* Hold down GP0 upon boot to enable only-keyboard mode