Creating the initial version of readme.md file

[sarangan12]

In the SDK Generation Process, we normally use a readme.md file in our JS Repository (usually under the swagger folder). After discussion with @qiaozha, we should be generating this file automatically (as part of generate-metadata). So, this file will be created the first time. Then, this file can be used/updated per requirement by the developer. This feature is required for the migration of management plane SDKs.

[ramya-rao-a]

As per the discussion in https://github.com/Azure/azure-sdk-for-js/pull/18398#discussion_r737881972, once we have the this initial readme file, we should add the generate:client npm script in package.json file