[docs] Update in-memory cache warm up (#7143)

Author: smyrick

docs/source/routing/performance/caching/in-memory.mdx

@@ -80,6 +80,19 @@ then look at `apollo.router.schema.loading.time` and `apollo.router.query_planni
 
 If the router is using distributed caching for query plans, the warm-up phase will also store the new query plans in Redis. Since all Router instances might have the same distributions of queries in their in-memory cache, the list of queries is shuffled before warm-up, so each Router instance can plan queries in a different order and share their results through the cache.
 
+<MinVersion version="1.61.0">
+
+#### Cache warm-up with headers
+
+</MinVersion>
+
+With router v1.61.0+  and v2.x+, if you have enabled exposing query plans via `--dev` mode or `plugins.experimental.expose_query_plan: true`, you can pass the `Apollo-Expose-Query-Plan` header to return query plans in the GraphQL response extensions. You must set the header to one of the following values:
+
+- `true`: Returns a human-readable string and JSON blob of the query plan while still executing the query to fetch data.
+- `dry-run`: Generates the query plan and aborts without executing the query.
+
+After using `dry-run`, query plans are saved to your configured cache locations. Using real, mirrored, or similar to production operations is a great way to warmup the caches before transitioning traffic to new router instances.
+
 ## Caching automatic persisted queries (APQ)
 
 [Automatic Persisted Queries (**APQ**)](/apollo-server/performance/apq/) enable GraphQL clients to send a server the _hash_ of their query string, _instead of_ sending the query string itself. When query strings are very large, this can significantly reduce network usage.