.NET: docs: updates the contributing information for CFS users

.gitignore

@@ -249,6 +249,9 @@ python/dotnet-ref
 dotnet/filtered-*.slnx
 **/*.lscache
 
+# Local .NET build overrides
+/dotnet/Directory.Build.rsp
+
 # Local tool state
 .omc/
 .omx/

CONTRIBUTING.md

@@ -167,6 +167,18 @@ Each language has its own dev setup guide, coding standards, and build scripts:
     - Integration tests: `dotnet test --filter-query "/*IntegrationTests*/*/*/*"` (requires API keys/endpoints)
     - Linting (auto-fix): `dotnet format`
 
+#### Microsoft Internal Feed Proxy for GitHub Copilot SDK (.NET)
+
+Microsoft contributors can route GitHub Copilot SDK npm downloads through the internal proxy without passing extra `dotnet` arguments:
+
+1. Create `dotnet/Directory.Build.rsp` with:
+
+   ```text
+   -p:CopilotNpmRegistryUrl=https://packagefeedproxy.microsoft.io/npm/
+   ```
+
+When running `dotnet build` (or other `dotnet` commands) from the `./dotnet` directory, this property is applied automatically.
+
 ### PR - CI Process
 
 The continuous integration (CI) system will automatically perform the required

dotnet/.gitignore

@@ -83,8 +83,6 @@ StyleCopReport.xml
 *.pgc
 *.pgd
 *.rsp
-# but not Directory.Build.rsp, as it configures directory-level build defaults
-!Directory.Build.rsp
 *.sbr
 *.tlb
 *.tli