docs: improve overall documentation

.github/readme/partials/documentation/setup/shared.md

@@ -0,0 +1,19 @@
+# 💕 Using the shared instance (~1 min setup, but with limitations)
+
+For convenience, you can use the shared instance available at [metrics.lecoq.io](https://metrics.lecoq.io) without any additional setup.
+
+```markdown
+![Metrics](https://metrics.lecoq.io/my-github-user)
+```
+
+This is mostly intended for previews, to enjoy all features consider using GitHub Action instead.
+Availability is not guaranteed.
+
+To ensure service availability, shared instance has a few limitations:
+  * Images are cached for 1 hour
+    * Rendered metrics **won't be updated** during this time window when queried
+    * You can manually update rendering again your metrics on [metrics.lecoq.io](https://metrics.lecoq.io)
+  * A rate limiter is enabled to prevent denial of service (it doesn't affect already cached metrics)
+  * Some plugins may not be available
+
+Service is provided free of charge, so please be gentle with it 🙂