Document usage expectations in the README

[ctgraham]

In the readme, we should document the integration process with Alma and Authorize.net:

• Alma / Primo
  • Reference these instructions: https://knowledge.exlibrisgroup.com/Primo/Product_Documentation/020Primo_VE/Primo_VE_(English)/060Library_Card_Configuration/Configuring_the_Pay_Fine_Link_for_Primo_VE
  • Note the requirement to append a ? to the URL to pass the jwt successfully.
  • Document required API permissions ("Users - Production Read/write")
• Authorize.net
  • Point to documentation for establishing / locating the API keys
  • Point to documentation for establishing / using the web service callback