Doc/update openapi readme (#335)

cmd/protoc-gen-openapi/README.md

@@ -6,9 +6,8 @@ Protocol Buffer service.
 
 Installation:
 
-        go install github.com/google/gnostic/cmd/protoc-gen-openapi
-
-
+    go install github.com/google/gnostic/cmd/protoc-gen-openapi
+
 Usage:
 
 	protoc sample.proto -I. --openapi_out=.
@@ -17,3 +16,50 @@ This runs the plugin for a file named `sample.proto` which
 refers to additional .proto files in the same directory as
 `sample.proto`. Output is written to the current directory.
 
+## options
+
+1. `version`: version number text, e.g. 1.2.3
+   - **default**: `0.0.1`
+2. `title`: name of the API
+   - **default**: empty string or service name if there is only one service
+3. `description`: description of the API
+   - **default**: empty string or service description if there is only one service
+4. `naming`: naming convention. Use "proto" for passing names directly from the proto files
+   - **default**: `json`
+   - `json`: will turn field `updated_at` to `updatedAt`
+   - `proto`: keep field `updated_at` as it is
+5. `fq_schema_naming`: schema naming convention. If "true", generates fully-qualified schema names by prefixing them with the proto message package name
+   - **default**: false
+   - `false`: keep message `Book` as it is
+   - `true`: turn message `Book` to `google.example.library.v1.Book`, it is useful when there are same named message in different package
+6. `enum_type`: type for enum serialization. Use "string" for string-based serialization
+   - **default**: `integer`
+   - `integer`: setting type to `integer`
+      ```yaml
+      schema:
+        type: integer
+        format: enum
+      ```
+   - `string`: setting type to `string`, and list available values in `enum`
+      ```yaml
+      schema:
+        enum:
+          - UNKNOWN_KIND
+          - KIND_1
+          - KIND_2
+        type: string
+        format: enum
+      ```
+7. `depth`: depth of recursion for circular messages
+   - **default**: 2, this depth only used in query parameters, usually 2 is enough
+8. `default_response`: add default response. If "true", automatically adds a default response to operations which use the google.rpc.Status message.
+   Useful if you use envoy or grpc-gateway to transcode as they use this type for their default error responses.
+   - **default**: true, this option will add this default response for each method as following:
+      ```yaml
+      default:
+        description: Default error response
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/google.rpc.Status'
+      ```