docs: document referencing CommandHook via env variable

README.md

@@ -11,6 +11,29 @@ depends on it.
 
 ## For mod development
 
+Set the `COMMANDHOOK` environment variable to the folder containing
+`CommandHook.dll` (i.e. wherever you've built CommandHook), the same way
+`VINTAGE_STORY` points at your game install.
+
+**Linux/macOS** (add to `~/.bashrc`, `~/.zshrc`, etc.):
+```bash
+export COMMANDHOOK="/path/to/CommandHook/CommandHook/bin/Release"
+```
+
+**Windows (PowerShell):**
+```powershell
+[Environment]::SetEnvironmentVariable("COMMANDHOOK", "C:\path\to\CommandHook\CommandHook\bin\Release", "User")
+```
+
+Then add a `Reference` to your `.csproj` next to your other game references:
+
+```xml
+<Reference Include="CommandHook">
+  <HintPath>$(COMMANDHOOK)/CommandHook.dll</HintPath>
+  <Private>false</Private>
+</Reference>
+```
+
 Add it as a dependency in your `modinfo.json`:
 
 ```json