docs(README): add managedBy to monitors.create example

The Quick Start example omitted the `managedBy` field, which the API
requires. Every fresh user copying the snippet from PyPI got
`DevhelmValidationError: managedBy: Field required` on their first
SDK call (verified by a fresh-user devex agent run against prod).

Adds `"managedBy": "DASHBOARD"` with an inline explainer comment
so users understand what the field means and when to override it.

Mirrors the same fix being shipped to docs.devhelm.io and to the
TypeScript SDK README. The cleaner long-term answer is server-side
defaulting from the X-DevHelm-Surface header — tracked separately.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: caballeto

README.md

@@ -31,6 +31,11 @@ monitor = client.monitors.create({
     "config": {"url": "https://api.example.com/health", "method": "GET"},
     "frequencySeconds": 60,
     "regions": ["us-east"],
+    # `managedBy` records who reconciles drift on this resource. Use
+    # "DASHBOARD" (the default for one-off SDK scripts), "CLI" if the
+    # monitor lives in a `devhelm.yml` you re-deploy, or "TERRAFORM"
+    # if it lives in `.tf` you re-apply.
+    "managedBy": "DASHBOARD",
 })
 
 # Get a single monitor