docs(contributing): state the definition-of-done principle before Pins (#132)

The Pins section codifies how example version pins move, but not the
broader principle it implies: a user-facing fix is not done at merge to
main. Examples run the published @subsquid/ponder package pinned in each
package.json/e2e.json, not the repo working tree — so a fix is neither
user-reproducible nor visible in example behavior until a version
carrying it is published and the pins advance through the release flow,
green under the examples e2e gate.

Add a short "Definition of done" subsection ahead of Pins that states
this crisply and leans on the existing Pins mechanics rather than
restating them.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dzhelezov

CONTRIBUTING.md

@@ -30,6 +30,15 @@ E2E_RPC_URL_1=<archive-rpc> node scripts/examples-e2e.mjs euler-subgraph
 The harness never prints RPC URLs. If the env var is unset, the example's public default RPC remains
 in use.
 
+## Definition of done
+
+A user-facing fix is not done when it merges to `main`. The examples run the **published**
+`@subsquid/ponder` package pinned in each `package.json`/`e2e.json`, not the repo working tree, so a
+fix on `main` is neither reproducible by a user nor visible in example behavior until a version
+carrying it is published to npm and the example pins have advanced to that version through the
+release flow (see below). Merged to `main` is not user-safe; a version published and pinned, green
+under the examples e2e gate, is.
+
 ## Pins
 
 Each example pins an exact `@subsquid/ponder` version in both `package.json` and `e2e.json`.