feat(openapi): Add modified typescript-fetch template

This template is modified from the original one, to handle multiple
status-code handling of response.

Don't ask me how they work, I don't quite understand them in depth...

openapi-template/README.mustache

@@ -0,0 +1,127 @@
+# {{npmName}}@{{npmVersion}}
+
+A TypeScript SDK client for the {{host}} API.
+
+## Usage
+
+First, install the SDK from npm.
+
+```bash
+npm install {{npmName}} --save
+```
+
+Next, try it out.
+
+{{#apiInfo}}{{#apis}}{{#-first}}{{#operations}}{{#operation}}{{#-first}}
+```ts
+{{>api_example}}
+```
+{{/-first}}{{/operation}}{{/operations}}{{/-first}}{{/apis}}{{/apiInfo}}
+
+## Documentation
+
+### API Endpoints
+
+All URIs are relative to *{{basePath}}*
+
+| Class | Method | HTTP request | Description
+| ----- | ------ | ------------ | -------------
+{{#apiInfo}}{{#apis}}{{#operations}}{{#operation}}*{{classname}}* | [**{{operationId}}**]({{apiDocPath}}/{{classname}}.md#{{operationIdLowerCase}}) | **{{httpMethod}}** {{path}} | {{summary}}
+{{/operation}}{{/operations}}{{/apis}}{{/apiInfo}}
+
+### Models
+
+{{#models}}{{#model}}- [{{{classname}}}]({{modelDocPath}}/{{{classname}}}.md){{/model}}
+{{/models}}
+
+### Authorization
+
+{{^authMethods}}Endpoints do not require authorization.{{/authMethods}}
+{{#hasAuthMethods}}Authentication schemes defined for the API:{{/hasAuthMethods}}
+{{#authMethods}}
+<a id="{{name}}{{#isOAuth}}-{{flow}}{{/isOAuth}}"></a>
+#### {{name}}{{#isOAuth}} {{flow}}{{/isOAuth}}
+
+{{#isApiKey}}
+
+- **Type**: API key
+- **API key parameter name**: `{{keyParamName}}`
+- **Location**: {{#isKeyInQuery}}URL query string{{/isKeyInQuery}}{{#isKeyInHeader}}HTTP header{{/isKeyInHeader}}
+{{/isApiKey}}
+{{#isBasicBasic}}
+
+- **Type**: HTTP basic authentication
+{{/isBasicBasic}}
+{{#isBasicBearer}}
+
+- **Type**: HTTP Bearer Token authentication{{#bearerFormat}} ({{{.}}}){{/bearerFormat}}
+{{/isBasicBearer}}
+{{#isHttpSignature}}
+
+- **Type**: HTTP signature authentication
+{{/isHttpSignature}}
+{{#isOAuth}}
+
+- **Type**: OAuth
+- **Flow**: {{flow}}
+- **Authorization URL**: {{authorizationUrl}}
+- **Scopes**: {{^scopes}}N/A{{/scopes}}
+{{#scopes}}  - `{{scope}}`: {{description}}
+{{/scopes}}
+{{/isOAuth}}
+{{/authMethods}}
+
+## About
+
+This TypeScript SDK client supports the [Fetch API](https://fetch.spec.whatwg.org/)
+and is automatically generated by the
+[OpenAPI Generator](https://openapi-generator.tech) project:
+
+- API version: `{{appVersion}}`
+- Package version: `{{npmVersion}}`
+{{^hideGenerationTimestamp}}
+- Build date: `{{generatedDate}}`
+{{/hideGenerationTimestamp}}
+- Generator version: `{{generatorVersion}}`
+- Build package: `{{generatorClass}}`
+
+The generated npm module supports the following:
+
+- Environments
+  * Node.js
+  * Webpack
+  * Browserify
+- Language levels
+  * ES5 - you must have a Promises/A+ library installed
+  * ES6
+- Module systems
+  * CommonJS
+  * ES6 module system
+
+{{#infoUrl}}
+For more information, please visit [{{{infoUrl}}}]({{{infoUrl}}})
+{{/infoUrl}}
+
+## Development
+
+### Building
+
+To build the TypeScript source code, you need to have Node.js and npm installed.
+After cloning the repository, navigate to the project directory and run:
+
+```bash
+npm install
+npm run build
+```
+
+### Publishing
+
+Once you've built the package, you can publish it to npm:
+
+```bash
+npm publish
+```
+
+## License
+
+[{{licenseInfo}}]({{{licenseUrl}}})