Authentication Error when adding a BCS

[Azure template for SharePoint 2019 / 2016 / 2013](https://github.com/Azure/azure-quickstart-templates/tree/master/sharepoint-adfs)

### Issue Details
Authenticating to a BCS is not inherently set up and I use an extra config step to improve the Claims Authentication. I have the work around added to the MD and looking to add a PR for review

Proposed change to README.md:
> **Notes:**
> - I strongly recommend to update SharePoint to a recent build just after the provisioning is complete.
> - With the default setting for virtual machines, provisioning of the template takes about 1h to complete.
> - The password complexity check in the form is not accurate and may validate a password that will be rejected by Azure when it provisions the VMs. Make sure to **use at least 2 special characters for the passwords**.
> - To ensure proper choice of windows authentication vs adfs please navigate on the farm server:
> 1. Open Central Administration
> 1. Manage web applications
> 1. Select SharePoint - 80 (or whatever the default web app port is)
> 1. Click Authentication Providers in the top bar
> 1. Scroll down ~3/4 of the way and find Sign in Page URL and select Custom Sign In Page
> 1. Add /\_trust/ to the custom sign in page text field -- this is the glue ADFS and service applications
> 1. click save

### Repro steps
1. Deploy Arm
2. Login in as Admin
3. Start BCS
4. Add BCS
5. Try to make changes to the BCS

Review the README.md in the linked SharePoint Azure template and compare the proposed notes with the authentication repro steps. Done means the README documents the BCS and Claims Authentication workaround and setup guidance described in the issue.

Written by the indexing model from the issue text.