Updated README

Author: rcastley

workshop/aws/sfx/README.md

@@ -2,6 +2,7 @@
 
 Set the following variables:
 
+- `aws_profile`: The AWS profile to use. This is the profile that you have set up in your `~/.aws/credentials` file.
 - `aws_region`: Which region do you want the instances in?
 - `signalfx_realm`: Which Observability Cloud realm to send to?
 - `signalfx_api_access_token`: A Observability Cloud user API access token. *Not* an org token. The user needs to be an admin.
@@ -12,6 +13,7 @@ Sample command:
 ```bash
 terraform apply \
 -auto-approve \
+-var="aws_profile=YOURPROFILE" \
 -var="aws_region=eu-central-1" \
 -var="signalfx_realm=eu0" \
 -var="signalfx_api_access_token=YOURTOKENHERE" \