Write a guide about entity level caching in graphql.

docs/guides/Entity-level-caching.md

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+# Entity-Level Caching with Tailcall
+Tailcall allows caching responses at the entity level to improve performance and reduce load on upstream services. 
+Here's how to implement entity caching with Tailcall:
+
+## Enable Caching on Type
+To cache a type, use the @cache directive on the type definition. This will cache all fields for the type:
+
+```type User @cache(maxAge: 900) {
+  id: Int
+  name: String 
+}
+```
+
+The maxAge specifies the maximum age of the cache in seconds.
+
+## Enable Caching on Individual Fields
+You can also enable caching on individual fields instead of the whole type:
+
+```type User {
+  id: Int
+  name: String @cache(maxAge: 300) 
+}
+```
+
+This caches just the name field for 300 seconds.
+
+## Caching with Arguments
+For fields with arguments, each unique combination of arguments will get its own cache entry:
+
+```type Query {
+  user(id: Int): User @cache(maxAge: 300)
+}
+```
+
+This caches the user field separately for each id value.
+
+## Cache Invalidation
+The cache is automatically invalidated when the maxAge expires. You can also invalidate the cache manually by calling purge() on the cache instance:
+
+```import { cache } from "@tailcall/graphql";
+
+async function invalidateCache() {
+  await cache.purge({
+    type: "User", 
+    id: { id: 10 }
+  });
+}
+```
+
+This purges the cache for the User with id 10.
+
+## Caching Lists
+Caching also works on lists of objects. The entire list is cached as one entry:
+
+```type Query {
+  users: [User] @cache(maxAge: 300)
+}
+```
+
+## Summary
+Use @cache on types or fields to enable caching
+Set a maxAge for automatic expiration
+Cache keys are based on field arguments
+Purge cache manually as needed
+Caching works on object lists