Add notice for deprecated IBM Watson endpoint.

[rahulsprajapati]

Description of the Change

Add a notice in readme and to Language Processing settings for deprecated IBM Watson endpoint.

Alternate Designs

Benefits

When user set deprecated endpoint for IBM Watson NLU API URL with watsonplatform.net in url, they'll see a notice about endpoint is deprecated and a link to guide on how to use/get new url.

Possible Drawbacks

Verification Process

1. Go to Language Processing Settings.

2. Add API url with watsonplatform.net endpoint.

3. You'll see an error/notice deprecated endpoint message as shown in below screenshot:

   

4. Update this URL with valid pattern i.e api.us-south.natural-language-understanding.watson.cloud.ibm.com

5. You won't see this deprecated endpoint message anymore.

   

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes: #316

Changelog Entry

• Add notice for deprecated IBM Watson endpoint.