[BE] Add readme for AWS runners (#5319)

terraform-aws-github-runner/README.md

@@ -0,0 +1,26 @@
+# Pytorch Terraform AWS Github Runner
+
+This is a terraform module that sets up self hosted github runners on AWS along with the infra needed to autoscale them
+
+# Directories
+
+```
+├── modules
+|   ├── download-lambda
+|   ├── runner-binaries-syncer # AWS Lambda func that
+|   ├── runners                # AWS Lambda func that scales runners up and down based
+|   |                          #   on SQS Events
+|   ├── runners-instances      # Defines how instances hosting new runners will be configured
+|   ├── setup-iam-permissions  # See Readme in the folder
+|   └── webhook                # AWS Lambda func that receives Github
+|                              #   webhooks and generates SQS events that
+|                              #   trigger scale up requests
+├── policies
+├── templates
+└── terraform_migrations
+
+```
+
+# Runner Architecture diagram
+This diagram shows how the runners function once deployed to AWS
+![High level runner architecture diagram](architecture-diagram.png)