adds depriciation message

Author: ninadhatkar

README.md

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 **Note**: This version of Contentstack CLI can be used only for v3 stacks. Use cli v1.x, for v2 stacks.
 
+**Note**: The contentstack-express framework CLI will soon be deprecated. However, our new CLI has many utilities that will help you manage your Contentstack account from the command line. Check out our [CLI documentation](https://www.contentstack.com/docs/developers/cli) for more details and get started with using our new CLI.
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 Run the following command in a Terminal or Command Prompt to globally install the latest version of Contentstack CLI on your system:
 
 ```bash