Add support for JSON manifest

[Rick-Kirkham]

Change Description:

This updates the conversion file to support the JSON manifest. The JSON manifest is updated to use placeholders in place of the port and app ID, just as the XML manifest does.

Also removes paragraph from the home page that is no longer applicable to this sample. Fixes 108

THESE CHANGES WON'T BE SUFFICIENT TO SUPPORT THE JSON MANIFEST. IT WILL ALSO BE NECESSARY TO UPDATE THE office-addin-sso TOOL SO THAT IT EDITS THE JSON MANIFEST THE WAY IT EDITS THE XML MANIFEST.

1. Do these changes impact any npm scripts commands (in package.json)? (e.g., running 'npm run start') If Yes, briefly describe what is impacted. NO

2. Do these changes impact VS Code debugging options (launch.json)? If Yes, briefly describe what is impacted. NO

3. Do these changes impact template output? (e.g., add/remove file, update file location, update file contents) If Yes, briefly describe what is impacted. NO

4. Do these changes impact documentation? (e.g., a tutorial on https://docs.microsoft.com/en-us/office/dev/add-ins/overview/office-add-ins) If Yes, briefly describe what is impacted. NO

If you answered yes to any of these please do the following:

Include 'Rick-Kirkham' in the review Make sure the README file is correct

Validation/testing performed:

I manually did all the work of the office-addin-sso tool and tested locally. Logged me in and ran just fine.