Add README to crates-io-logs module

Author: jdno

terragrunt/modules/crates-io-logs/README.md

@@ -0,0 +1,36 @@
+# Infrastructure to Count Crate Downloads
+
+This module creates the infrastructure that enables [crates.io] to count crate
+downloads asynchronously using request logs from our Content Delivery Networks.
+
+Whenever a new archive with request logs is uploaded to S3, S3 pushes an event
+into a SQS queue. [crates.io] monitors the queue and processes incoming events.
+From the event, it can determine what files to fetch from S3, download and then
+parse them, and update the download counts in the database.
+
+```mermaid
+sequenceDiagram
+	static.crates.io ->> S3: Uploads logs
+	S3 ->> SQS: Queues event
+	crates.io ->> SQS: Pulls event from queue
+	crates.io ->> S3: Fetches new log file
+	crates.io ->> crates.io: Parses log file
+	crates.io ->> crates.io: Updates download counts
+```
+
+See [rust-lang/simpleinfra#372] for a detailed discussion of the design.
+
+## AWS Accounts
+
+The infrastructure for [crates.io] has historically been deployed to the
+`legacy` AWS account. For this infrastructure, new accounts have been created
+that follow the new convention of specialized and isolated accounts for
+services.
+
+This requires the S3 bucket with the request logs in the `legacy` account to
+push events into the SQS queue in a different account. And the [crates.io]
+application needs a second set of AWS credentials to pull events from the
+queue.
+
+[crates.io]: https://crates.io
+[rust-lang/simpleinfra#372]: https://github.com/rust-lang/simpleinfra/issues/372