Bump version to 1.0.0-alpha.11 (#774)

README.md

@@ -41,7 +41,7 @@ necessary targets:
 
 ```swift
 dependencies: [
-  .package(url: "https://github.com/grpc/grpc-swift.git", from: "1.0.0-alpha.9")
+  .package(url: "https://github.com/grpc/grpc-swift.git", from: "1.0.0-alpha.11")
 ]
 ```