Add info about dummy dependencies

docs/building/internals.md

@@ -16,6 +16,24 @@ We are calling `dotnet` tool build for `$Top` directory
 - `src\powershell` for CoreCLR builds (all platforms)
 - `src\Microsoft.PowerShell.ConsoleHost` for FullCLR builds (Windows only)
 
+
+### Dummy dependencies
+
+We use dummy dependencies between project.json files to leverage `dotnet` build functionality.
+For example, `src\Microsoft.PowerShell.ConsoleHost\project.json` has dependency on `Microsoft.PowerShell.PSReadLine`,
+but in reality, there is no build dependency.
+
+Dummy dependencies allows us to build just `$Top` folder, instead of building several folders.
+
+### Dummy dependencies rules
+
+* If assembly is part of FullCLR build,
+it should be listed as a dependency for FullCLR $Top folder (src\Microsoft.PowerShell.ConsoleHost)
+
+* If assembly is part of CoreCLR build,
+it should be listed as a dependency for CoreCLR $Top folder (src\powershell)
+
+
 Modules
 ----------