update README for admin next development

[kevinhughes27]

I've updated the readme with new documentation for working on Admin Next. The development of the new admin client hasn't been as clean and easy to follow as I'd hoped but it is starting to come together. The last few PRs have been reasonable patches and the patterns are starting to mature.

• Admin Next Team Create #820 create pattern WIP
• update render helper to abstract more #817 refactor some rendering code
• Division show #816 show page pattern
• Game update score #812 show + mutation

I'm still tracking the overall project in #750. Here are a few good first issues if anyone is interested:

• Adding the settings page and update setting mutation
• Porting search and filtering to the index pages
• Porting the player app page

I'm going to be finishing the create pattern, the map + field page and working on the new CSV import.

@patrickkenzie @JRSpencer @RichardBlair