Update get-started.md | Removes deprecation warning for webpackconfig in favor of generateWebpackConfig

[chmod600]

Summary

Using webpackconfig throws a deprecation warning and encourages user to use generateWebpackConfig() instead. Updates docs to avoid this deprecation warning.

Other Information

Output: 

 $ ./bin/shakapacker-dev-server
⚠️
DEPRECATION NOTICE:
The 'webpackConfig' is deprecated and will be removed in a future version.
Please use 'globalMutableWebpackConfig' instead, or use
'generateWebpackConfig()' to avoid unwanted config mutation across the app.

For more information, see version 7 upgrade documentation at:
https://github.com/shakacode/shakapacker/blob/master/docs/v7_upgrade.md

Pull Request checklist

• [x ] Add/update test to cover these changes
• [ x] Update documentation
• [ x] Update CHANGELOG file

[G-Rath]

looks good to me!

[G-Rath]

not blocking, but fwiw this can be simplified further as generateWebpackConfig now takes a config to be merged in:

module.exports = generateWebpackConfig({
    plugins: [new ForkTsCheckerWebpackPlugin()]
});