improve github app installation

README.md

@@ -53,7 +53,7 @@ At the moment all projects are visible without authentication.
 
 For some actions a login is required. This login can either be based on GitHub
 or on Gitea (more logins may follow). The backend is set by the
-`services.buildbot-nix.master.authBackend` NixOS option.
+`services.buildbot-nix.master.authBackend` NixOS option ("gitea"/"github", "github" by default).
 
 We have the following two roles:
 
@@ -68,18 +68,25 @@ We have the following two roles:
 
 #### GitHub App
 
-This is the preferred option to setup buildbot-nix.
+This is the preferred option to setup buildbot-nix for GitHub.
 
 To integrate with GitHub using app authentication:
 
-1. **GitHub App**: Set up a GitHub app for Buildbot to enable GitHub user
-   authentication on the Buildbot dashboard. Enable the following permissions:
-   - Contents: Read-only
-   - Metadata: Read-only
-   - Commit statuses: Read and write
-   - Webhooks: Read and write
+1. **GitHub App**:
+    1. Create a new GitHub app by navigating to `https://github.com/settings/apps/new` for single-user installations or `https://github.com/organizations/<org>/settings/apps/new` for organisations where `<org>` is the name of your GitHub organizaction.
+    2. GitHub App Name: "buildbox-nix <org>"
+    3. Homepage URL: `https://buildbot.<your-domain>`
+    4. Callback URL: `https://buildbot.<your-domain>/auth/login`.
+    5. Disable the Webhook
+    6. Repository Permissions:
+      - Contents: Read-only
+      - Commit statuses: Read and write
+      - Metadata: Read-only
+      - Webhooks: Read and write
 2. **GitHub App private key**: Get the app private key and app ID from GitHub,
    configure using the buildbot-nix NixOS module.
+    - Set `services.buildbot-nix.master.github.authType.app.id = <your-github-id>;`
+    - Set `services.buildbot-nix.master.github.authType.app.secretKeyFile = "/path/to.pem";`
 3. **Install App**: Install the app for an organization or specific user.
 4. **Refresh GitHub Projects**: Currently buildbot-nix doesn't respond to
    changes (new repositories or installations) automatically, it is therefore
@@ -184,3 +191,4 @@ The following instances integrated with Gitea:
 
 We have a matrix channel at
 [buildbot-nix](https://matrix.to/#/#buildbot-nix:thalheim.io).
+