Make languageCodenames optional for config

[CollierCZ]

Motivation

To simplify configuration for simple, single-language projects -- not require them to add additional options when getting started.

Proposed solution

Make languageCodenames optional in gatsby-config.js with a fallback to default if not provided.

[Simply007]

Also add the note to the Readme.

[Hardik0307]

@makma Want to work on this.

[makma]

Hello @Hardik0307, welcome to the Kentico community! I've already sent you an invite to the organization and assigned you the issue. Happy coding! 🎉

[Simply007]

Hello @Hardik0307, let me welcome you to Kentico Community as well. I assume you are participating in Hacktoberfest.

I have set "hacktoberfest" topic to this repository so that your pull request would be counted once it is reviewed and merged.