Add cross-compilation toolchain to docs (#2172)

Author: samfreund

docs/source/docs/contributing/building-photon.md

@@ -18,6 +18,10 @@ This section contains the build instructions from the source code available at [
 
  [pnpm](https://pnpm.io/) is the package manager used to download dependencies for the UI. To install pnpm, follow [the instructions on the official pnpm website](https://pnpm.io/installation).
 
+**Cross-Compilation Toolchains (Optional):**
+
+ If you plan to deploy PhotonVision to a coprocessor like a Raspberry Pi, you will need to install the appropriate cross-compilation toolchain for your platform. For `linuxarm64` devices, this can be accomplished by running `./gradlew installArm64Toolchain` in the root folder of the project.
+
 ## Compiling Instructions
 
 ### Getting the Source Code