add proto/ with prototool management + backend/proto and backend/identity example

proto/README.md

@@ -0,0 +1,29 @@
+# Backstage proto
+
+Collection of all Backstage protobuf definitions.
+
+## Usage
+
+Managed with prototool, install instructions at https://github.com/uber/prototool/blob/dev/docs/install.md
+
+### Generate code
+
+Run to generate code to `backend/proto/`:
+
+```
+$ prototool generate
+```
+
+Generated code should be check in to the repo.
+
+### Import generated go code
+
+Example import of inventory:
+
+```go
+package spotify.backstage.identity.v1;
+
+func main() {
+  identityv1.NewInventoryClient(nil)
+}
+```