Update SCIM docs to include section on SA auth (#1249)

This change adds a brief section to the SCIM api documentation
explaining how to use Org Level Service Accounts to make changes through
SCIM.

Author: niall-wb

content/guides/hosting/iam/scim.md

@@ -25,8 +25,18 @@ In addition, examples use user IDs such as `abc` and `def`. Real requests and re
 
 ## Authentication
 
+Access to the SCIM API can be authenticated in two ways:
+
+### Users
+
 An organization or instance admin can use basic authentication with their API key to access the SCIM API. Set the HTTP request's `Authorization` header to the string `Basic` followed by a space, then the base-64 encoded string in the format `username:API-KEY`. In other words, replace the username and API key with your values separated with a `:` character, then base-64-encode the result. For example, to authorize as `demo:p@55w0rd`, the header should be `Authorization: Basic ZGVtbzpwQDU1dzByZA==`.
 
+### Service accounts
+
+An organization service account with the `admin` role can access the SCIM API. The username is left blank and only the API key is used. Find the API key for service accounts in the **Service account** tab in the organization dashboard. Refer to [Organization-scoped service accounts]({{< relref "/guides/hosting/iam/authentication/service-accounts.md/#organization-scoped-service-accounts" >}}).
+
+Set the HTTP request's `Authorization` header to the string `Basic` followed by a space, then the base-64 encoded string in the format `:API-KEY` (notice the colon at the beginning with no username). For example, to authorize with only an API key such as `sa-p@55w0rd`, set the header to: `Authorization: Basic OnNhLXBANTV3MHJk`.
+
 ## User resource
 
 The SCIM user resource maps to W&B users.