docs: update readme for lockfile (#1220)

Author: AllanZhengYP

README.md

@@ -20,7 +20,8 @@ The new AWS SDK for JavaScript will also be able to run alongside the version 2.
 Let’s walk through setting up a project that depends on DynamoDB from the SDK and makes a simple service call. The following steps use yarn as an example. These steps assume you have Node.js and yarn already installed.
 
 1. Create a new Node.js project.
-2. Inside of the project, run: `yarn add @aws-sdk/client-dynamodb@gamma`
+2. Inside of the project, run: `yarn add @aws-sdk/client-dynamodb@gamma`. Adding packages results in update in [lock file](https://docs.npmjs.com/configuring-npm/package-locks.html), `yarn.lock` or `package-lock.json`. You **should** commit your lock file along with your code to avoid potential breaking changes.
+
 3. Create a new file called index.js, create a DynamoDB service client and send a request.
 
 ```javascript