New README for the Swift gRPC plugin.

Plugin/README.md

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+# Swift gRPC Plugin
+
+This directory contains the Swift gRPC plugin for `protoc`,
+the Protocol Buffer Compiler.
+
+It is built with the Swift Package Manager and the included
+Makefile. The resulting binary is named `protoc-gen-swiftgrpc`
+and can be called from `protoc` by adding the `--swiftgrpc_out`
+command-line option. For example, here's an invocation from
+the Makefile:
+
+	protoc ../Examples/Echo/echo.proto --proto_path=../Examples/Echo --swiftgrpc_out=. 
+
+The plugin uses template files in the [templates](templates) 
+directory. This directory currently must be in the same location 
+where the compiler is run.
+