add documentation to update package.json lint script to include .tsx files in react with typescript framework guide

[hkennyv]

Currently, the generated package.json for the "Typescript + Webpack" template uses the following lint script in the package.json:

    "lint": "eslint --ext .ts ."

I think it could be a good addition to the React framework integration guide to let the user know they should update this script to include the .tsx extension as well.

New package.json lint script:

    "lint": "eslint --ext .ts,.tsx ."