Added details + links to more context for private apps

[darius-gai]

Updated the Get API Access section to add more context for the new 'Enable private app development' feature

Warn Checklist

• [ ] This changes the interface and requires a Major/Minor version change.
• [ ] I have :tophat:'d these changes by using the commands I changed by hand.
• [ ] I have added a dependancy to the project.
• [ ] I have considered any potential impact on node-themekit