add private key for strong name assemblies

csharp/keys/Google.Protobuf.snk

diff --git a/csharp/keys/Google.Protobuf.snk b/csharp/keys/Google.Protobuf.snk
new file mode 100644
index 0000000..7515443
--- /dev/null
+++ b/csharp/keys/Google.Protobuf.snk
Binary files differ

csharp/keys/README.md

diff --git a/csharp/keys/README.md b/csharp/keys/README.md
index 1a01419..f6f27e8 100644
--- a/csharp/keys/README.md
+++ b/csharp/keys/README.md
@@ -2,4 +2,8 @@
 --------
 
 - Google.Protobuf.public.snk:
-  Public key to verify strong name of Google.Protobuf assemblies.
\ No newline at end of file
+  Public key to verify strong name of Google.Protobuf assemblies.
+- Google.Protobuf.snk:
+  Signing key to provide strong name of Google.Protobuf assemblies.
+  As per https://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.110).aspx
+  signing key should be checked into the repository.
\ No newline at end of file