docs(github-actions-usage): Add README

packages/github-actions-usage/README.md

@@ -0,0 +1,29 @@
+# GitHub Actions Usage
+
+This app, deployed as an AWS Lambda, tracks which GitHub Actions are in use across all repositories in the organisation.
+
+## How it works
+1. Get all records from the `github_workflows` table
+2. For each record returned, parse the `contents` field
+    1. If the field is empty, log a message
+    2. Use [Ajv](https://ajv.js.org/) to validate the `contents` field against a [schema](https://json.schemastore.org/github-workflow.json) published by https://www.schemastore.org/json/
+    3. If validation fails, log a message
+3. Extract the `uses` string from the `contents`
+4. Save results to database
+
+## Querying the data
+The Lambda writes to the [`github_actions_usage` table](../common/prisma/migrations/20240222201635_guardian_github_actions_usage/migration.sql).
+
+The [`view_github_actions` SQL view](../common/prisma/migrations/20240222201635_guardian_github_actions_usage/migration.sql) also exists
+to make it easier to query the data.
+This view shows the archived status of a repository, the name of the Action being used, and the version.
+
+## Updating the schema
+The [schema](./src/schema/github-workflow.json) might need updating from time to time.
+
+To do this, run the following, and commit the resulting file:
+
+```bash
+cd /path/to/github-actions-usage
+wget --output-document src/schema/github-workflow.json https://json.schemastore.org/github-workflow.json
+```