add instructions in doc/building.md

doc/building.md

@@ -1,6 +1,8 @@
+# Building
+
 The following instructions are only relevant for those who want to develop the orca runtime or orca cli tool.
 
-### Requirements
+## Requirements
 
 All of the installation requirements for regular users also apply for developers, with these additions:
 
@@ -11,9 +13,42 @@ All of the installation requirements for regular users also apply for developers
 - Xcode command-line tools (Mac only)
 	- These can be installed with `xcode-select --install`.
 
-### Building 
+## Building from source
+
+
+First clone the orca repo: `git clone https://github.com/orca-app/orca.git`
+
+#### Angle and Dawn
+
+Orca depends on Angle for OpenGL ES, and Dawn for WebGPU. You can either build them locally, or grab a precompiled binary built in CI:
+
+- To build locally: 
+	- `cd` to the orca directory 
+	- run `./orcadev build-angle --release`
+	- run `./orcadev build-dawn --release`
+
+- To use a precompiled binary: 
+	- Go to [https://github.com/orca-app/orca/actions/workflows/build-all.yaml](https://github.com/orca-app/orca/actions/workflows/build-all.yaml)
+	- Download the artifacts from a previous run. 
+		- On ARM macs, download `angle-mac-arm64` and `dawn-mac-arm64`.
+		- On Intel macs, download `angle-mac-x64` and `dawn-mac-x64`.
+		- On Windows, download `angle-windows-x64` and `dawn-windows-x64`.  
+	- Unzip the artifacts and put their content in the `build` directory at the root of the orca repo (you can create that directory if it doesn't exist).
+
+You only need to do this once, until we change the Angle or Dawn versions we depend on.
+
+#### Building Orca
+
+- `cd` to the orca directory and run `./orcadev build` (macOS) or `orcadev build` (Windows)
+- If this is the first time you build orca, and you have skipped the previous section, this will print a message telling you you first need to build Angle and Dawn. 
+
+#### Installing a dev version of the tooling and SDK
+
+- Inside the repo, run `./orcadev install directory`. This will install a dev version of the tooling and SDK into `directory`. 
+- Make sure `directory` is in your `PATH` environment variable.
+
+You can then use this dev version normally through the `orca` command line tool.
 
-TODO
 
 ### FAQ