add invocation to README

README.md

@@ -16,6 +16,24 @@ Some areas we hope to improve over the old generator are:
 - simpler, faster implementation, and
 - better error reporting.
 
+Installation
+------------
+`go get github.com/googleapis/gapic-generator-go/cmd/protoc-gen-go_gapic`.
+If you are using Go 1.11 and see error `cannot find main module`, see this [FAQ page](https://github.com/golang/go/wiki/Modules#why-does-installing-a-tool-via-go-get-fail-with-error-cannot-find-main-module).
+
+The generator works as a `protoc` plugin, get `protoc` from [google/protobuf](https://github.com/protocolbuffers/protobuf).
+
+Invocation
+----------
+`protoc --go_gapic_out [OUTPUT_DIR] --go_gapic_opt 'package/path/url;name' a.proto b.proto`
+
+The `go_gapic_opt` flag is necessary because we need to know where to generated file will live.
+The substring before the semicolon is the import path of the package, e.g. `github.com/username/awesomeness`.
+The substring after the semicolon is the name of the package used in the `package` statement.
+Idiomatically the name is last element of the path but it need not be.
+For instance, the last element of the path might be the package's version, and the package would benefit
+from a more descriptive name.
+
 Disclaimer
 ----------
 This generator is currently experimental. Please don't use it for anything mission-critical.