Document plugin

Author: mraerino

README.md

@@ -1,5 +1,42 @@
 Netlify Build plugin identity-sso - Protect a site with SSO via identity.
 
-# Install
+## Install
 
-Please install this plugin from the Netlify app.
+The recommended way to install this right now is using a git npm dependency:
+
+`package.json`
+
+```json
+{
+  // ...
+  "dependencies": {
+    "netlify-plugin-identity-sso": "mraerino/build-plugin-identity-sso#semver:^0.1"
+  }
+}
+```
+
+`netlify.toml`
+
+```toml
+[[plugins]]
+package = "netlify-plugin-identity-sso"
+```
+
+### Identity Setup
+
+- Go to `Settings -> Identity` on your site and click `Enable Identity`
+- Add a third-party auth provider, e.g. Google
+- Setup the identity webhook:
+
+  - URL: `https://<site-name>.netlify.app/.netlify/functions/sso-auth`
+  - Secret: _Generate a long random string_
+  - Events: _Tick all boxes_
+
+- Set the webhook secret as a build environment variable `WEBHOOK_SECRET` for
+  use in the function
+
+## Gotchas
+
+- **Do not use a cached directory for publishing.** For some reason this breaks
+  writing of a custom `netlify.toml` as part of the build.
+- This is hardcoded to allow any user with a `@netlify.com` email address.