Add example for creating custom resources

custom-resources/.gitignore

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+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+
+# testing
+/coverage
+
+# production
+/build
+
+# misc
+.DS_Store
+
+# sst build output
+.build
+
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*

custom-resources/README.md

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+# How to create a REST API
+
+An example serverless app created with SST.
+
+## Getting Started
+
+[**Read the tutorial**](https://serverless-stack.com/examples/how-to-create-a-rest-api-with-serverless.html)
+
+Install the dependencies.
+
+```bash
+$ npm install
+```
+
+## Commands
+
+### `npm run start`
+
+Starts the Live Lambda Development environment.
+
+### `npm run build`
+
+Build your app and synthesize your stacks.
+
+### `npm run deploy [stack]`
+
+Deploy all your stacks to AWS. Or optionally deploy, a specific stack.
+
+### `npm run remove [stack]`
+
+Remove all your stacks and all of their resources from AWS. Or optionally removes, a specific stack.
+
+### `npm run test`
+
+Runs your tests using Jest. Takes all the [Jest CLI options](https://jestjs.io/docs/en/cli).
+
+## Documentation
+
+Learn more about the SST.
+
+- [Docs](https://docs.serverless-stack.com/)
+- [@serverless-stack/cli](https://docs.serverless-stack.com/packages/cli)
+- [@serverless-stack/resources](https://docs.serverless-stack.com/packages/resources)

custom-resources/lib/MyStack.js

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+import { Token } from "@aws-cdk/core";
+import * as sst from "@serverless-stack/resources";
+
+import Sum from "./Sum";
+
+export default class MyStack extends sst.Stack {
+  constructor(scope, id, props) {
+    super(scope, id, props);
+
+    // Create the custom resource
+    const sum = new Sum(this, "MySum", { lhs: 40, rhs: 2 });
+
+    this.addOutputs({
+      Result: Token.asString(sum.result),
+    });
+  }
+}

custom-resources/lib/Sum.js

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+import { Construct, CustomResource, Token } from "@aws-cdk/core";
+import * as cr from "@aws-cdk/custom-resources";
+import * as sst from "@serverless-stack/resources";
+
+export default class Sum extends Construct {
+  result;
+
+  constructor(scope, id, props) {
+    super(scope, id);
+
+    // Create Lambda function
+    const eventHandler = new sst.Function(this, "Handler", {
+      handler: "src/lambda.main",
+    });
+
+    // Create custom resource
+    const provider = new cr.Provider(this, "ResourceProvider", {
+      onEventHandler: eventHandler,
+    });
+
+    const resource = new CustomResource(this, "Resource", {
+      resourceType: "Custom::Sum",
+      serviceToken: provider.serviceToken,
+      properties: {
+        lhs: props.lhs,
+        rhs: props.rhs,
+      },
+    });
+
+    this.result = Token.asNumber(resource.getAtt("Result"));
+  }
+}

custom-resources/lib/index.js

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+import MyStack from "./MyStack";
+
+export default function main(app) {
+  new MyStack(app, "my-stack");
+
+  // Add more stacks
+}