Add pricing details to new section in readme

[jeffpaul]

Description of the Change

This PR pulls the pricing info for Azure from the Set Up section into a new Pricing section and includes details on Watson pricing. It also updates the naming of the Watson set up section from Content Tagging to Language Processing to better match the settings sections in the plugin itself.

Alternate Designs

We could skip a Pricing section and just include these notes in the Azure and Watson set up sections.

Benefits

Ensures that we best try to call out any costs that ClassifAI users may incur, but also note that for most average / low volume uses that ClassifAI's AI services could be totally free.

Possible Drawbacks

If someone reads these statements to be ALL ClassifAI AI service usage is free and then incurs costs, they may be 😡. So hopefully the phrasing in the Pricing section is clear enough to avoid that scenario.

Verification Process

Manually verified via the GitHub web UI.

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 #152.

Changelog Entry

n/a