WIP: Add basic build instructions

README.md

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+# gb-link-firmware
+## Wiki
+You can find all relevant information for the gb-link project in [the wiki](https://github.com/stacksmashing/gb-link-firmware/wiki)
+## How to build
+### Get the code
+Clone the gb-link-firmware repo using `git clone https://github.com/stacksmashing/gb-link-firmware.git` and get a copy of the pico-sdk using `git clone https://github.com/raspberrypi/pico-sdk`.
+
+In order to include the tinyusb source make sure to run `git submodule update --init` inside `./pico-sdk`.
+
+### Set up the pico-sdk depedencies
+For Ubuntu use 
+
+`sudo apt install cmake gcc-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib`
+
+and make sure to check the [official repo](https://github.com/raspberrypi/pico-sdk) for changes.
+
+Then copy `./pico-sdk/external/pico_sdk_import.cmake` into `./gb-link-firmware/` and make sure to set `PICO_SDK_PATH` to the location of the pic-sdk folder.
+
+### Set up build folder
+Inside `gb-link-firmware` create an `build` folder. If you made changes to the location of `pico-sdk`, the environment variable or similiar it is often easier to clean the content of this folder to make sure changes got pick up.
+
+### Actually compile something
+Inside `gb-link-firmware` run both `cmake ..` and `make` in order to get your firmware build.
+
+If you made no changes to the make file the firmware should be generated as `gbusb.uf2`.