Merge pull request #4 from DeterminateSystems/grahamc-patch-1

README updates

README.md

@@ -1,35 +1,78 @@
-# The Magic Nix Cache Action
+# Magic Nix Cache
 
-Cut CI time by 50% or more by caching to GitHub Actions' cache, for free. No configuration required.
+Save 30-50%+ of CI time without any effort or cost.
+Use Magic Nix Cache, a totally free and zero-configuration binary cache for Nix on GitHub Actions.
 
-<!--
+Add our [GitHub Action][action] after installing Nix, in your workflow, like this:
-cat action.yml| nix run nixpkgs#yq-go -- '[[ "Parameter", "Description", "Required", "Default" ], ["-", "-", "-", "-"]] + [.inputs | to_entries | sort_by(.key) | .[] | ["`" + .key + "`", .value.description, .value.required // "", .value.default // ""]] | map(join(" | ")) | .[] | "| " + . + " |"' -r
--->
 
 ```yaml
-name: Flake Check
+- uses: DeterminateSystems/magic-nix-cache-action@main
+```
+
+See [Usage](#usage) for a detailed example.
+
+## Why use the Magic Nix Cache?
+
+Magic Nix Cache uses the GitHub Actions [built-in cache][ghacache] to share builds between Workflow runs, and has many advantages over alternatives.
+
+1. Totally free: backed by GitHub Actions' cache, there is no additional service to pay for.
+1. Zero configuration: add our action to your workflow.
+   That's it.
+   Everything built in your workflow will be cached.
+1. No secrets: Forks and pull requests benefit from the cache, too.
+1. Secure: Magic Nix Cache follows the [same semantics as the GitHub Actions cache][semantics], and malicious pull requests cannot pollute your project.
+1. Private: The cache is stored in the GitHub Actions cache, not with an additional third party.
+
+> **Note:** the Magic Nix Cache doesn't offer a publicly available cache.
+> This means the cache is only usable in CI.
+> [Zero to Nix][z2n] has an article on binary caching if you want to [share Nix builds][z2ncache] with users outside of CI.
+
+## Usage
+
+Add it to your Linux and macOS GitHub Actions workflows, like this:
+
+```yaml
+name: CI
 
 on:
-  pull_request:
   push:
-    branches: [main]
+  pull_request:
 
 jobs:
-  flake-check:
+  check:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
-      - uses: DeterminateSystems/flake-checker-action@v4
+      - uses: DeterminateSystems/nix-installer-action@main
-        with:
+      - uses: DeterminateSystems/magic-nix-cache-action@main
-          fail-mode: true
+      - run: nix flake check
-      - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v4
-      - uses: DeterminateSystems/nix-installer-action-cache@focus-on-cache
-
-      - name: "Nix Flake Check"
-        run: nix flake check . -L
 ```
 
+That's it.
+Everything built in your workflow will be cached.
+
+## Usage Notes
+
+The GitHub Actions Cache has a rate limit on reads and writes.
+Occasionally, large projects or large rebuilds may exceed those rate-limits, and you'll see evidence of that in your logs.
+The error looks like this:
+
+```
+error: unable to download 'http://127.0.0.1:37515/<...>': HTTP error 418
+       response body:
+       GitHub API error: API error (429 Too Many Requests): StructuredApiError { message: "Request was blocked due to exceeding usage of resource 'Count' in namespace ''." }
+```
+
+The caching daemon and Nix both handle this gracefully, and won't cause your CI to fail.
+When the rate limit is exceeded while pulling dependencies, your workflow may perform more builds than usual.
+When the rate limit is exceeded while uploading to the cache, the remainder of those store paths will be uploaded on the next run of the workflow.
+
+
+# Action Options
+<!--
+cat action.yml| nix run nixpkgs#yq-go -- '[[ "Parameter", "Description", "Required", "Default" ], ["-", "-", "-", "-"]] + [.inputs | to_entries | sort_by(.key) | .[] | ["`" + .key + "`", .value.description, .value.required // "", .value.default // ""]] | map(join(" | ")) | .[] | "| " + . + " |"' -r
+-->
+
 | Parameter | Description | Required | Default |
 | - | - | - | - |
 | `diagnostic-endpoint` | Diagnostic endpoint url where diagnostics and performance data is sent. To disable set this to an empty string. |  | https://install.determinate.systems/magic-nix-cache/perf |