Add rudimentary installation instructions to plugin

Describe how `protoc` finds the plugin binary in the path. A simple
installation can be performed by copying the binary to a directory in
the path.

Also fix the example `protoc` invocation to match the Makefile by
adding an explicit `--plugin` argument.

Also add a few more files to .gitignore that are generated during
plugin build/test.

.gitignore

@@ -5,3 +5,5 @@ xcuserdata
 third_party/**
 Plugin/Packages/**
 Plugin/protoc-*
+Plugin/swiftgrpc.log
+Plugin/echo.*.swift

Plugin/README.md

@@ -6,11 +6,15 @@ 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:
+command-line option and `--plugin` option. For example, here's an
+invocation from the Makefile:
 
-	protoc ../Examples/Echo/echo.proto --proto_path=../Examples/Echo --swiftgrpc_out=. 
+		protoc ../Examples/Echo/echo.proto --proto_path=../Examples/Echo --plugin=./protoc-gen-swiftgrpc --swiftgrpc_out=.
 
 The plugin uses template files in the [Templates](Templates) directory. 
 These files are compiled into the `protoc-gen-swiftgrpc` plugin executable.
 
+The Swift gRPC plugin can be installed by placing the
+`protoc-gen-swiftgrpc` binary into one of the directories in your
+path.  Specifying `--swiftgrpc_out` to `protoc` will automatically
+search the `PATH` environment variable for this binary.