C#: Add readme to stub folder

Author: tamasvajk

csharp/ql/src/Stubs/README.md

@@ -0,0 +1,19 @@
+# Generate stubs
+
+Stubs can be generated from Nuget packages with the `make_stubs_nuget.py` script.
+
+The following calls generate stubs for `Newtonsoft.Json`:
+
+```
+python make_stubs_nuget.py Newtonsoft.Json
+python make_stubs_nuget.py Newtonsoft.Json latest
+python make_stubs_nuget.py Newtonsoft.Json 13.0.1
+python make_stubs_nuget.py Newtonsoft.Json 13.0.1 /Users/tmp/working-dir
+```
+
+The output stubs are found in the `[DIR]/output/stubs` folder and can be copied over to `csharp/ql/test/resources/stubs`.
+
+In some more involved cases the output files need to be edited. For example `ServiceStack` has Nuget dependencies, which
+are included in the `Microsoft.NETCore.App` framework stub. These dependencies generate empty packages, which can be
+removed. The `ProjectReference` entries referencing these removed empty packages also need to be deleted from the
+`.csproj` files.