Update DEVGUIDE.md (#16197)

DEVGUIDE.md

@@ -215,6 +215,17 @@ or
 > **Note**
 > Please note, that by default, **Release** version of IL baseline tests will be running in CI, so when updating baseline (.bsl) files, make sure to add `-c Release` flag to the build command.
 
+
+### Updating FCS surface area baselines
+
+```bash
+$env:TEST_UPDATE_BSL=1
+dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj --filter "SurfaceAreaTest" /p:BUILDING_USING_DOTNET=true
+dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj --filter "SurfaceAreaTest" /p:BUILDING_USING_DOTNET=true
+dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj --filter "SurfaceAreaTest" -c Release /p:BUILDING_USING_DOTNET=true
+dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj --filter "SurfaceAreaTest" -c Release /p:BUILDING_USING_DOTNET=true
+```
+
 ## Automated Source Code Formatting
 
 Some of the code in this repository is formatted automatically by [Fantomas](https://github.com/fsprojects/fantomas). To format all files use: