Add "Third-Party Plugins" section to the docs

[jkwade]

Several third-party gateways, shopping carts, and other plugs have built integrations into the Balanced API. Balanced should showcase them.

[jkwade]

Shopping Carts

• Drupal Commerce
• WordPress
• Easy Digital Downloads
• Cart66
• Mijreh
• Spree

Frameworks

• Meteor.js Package
• ActiveMerchant

Recurring Gateways

• Chargebee

Gateways

• Spreedly

Crowdfunding Gateways

• Crowdtilt API/Crowdhoster

Card Readers

• Cardflight

Accounting

• Subledger

[remear]

Added into 1.1