feat: derived loader docs (#404)

Author: saihaj

website/pages/en/developing/assemblyscript-api.mdx

@@ -298,6 +298,33 @@ if (transfer == null) {
 
 > Note: If there is no entity created in the given block, `loadInBlock` will return `null` even if there is an entity with the given ID in the store.
 
+#### Looking up derived entities
+
+As of `graph-node` v0.31.0, `@graphprotocol/graph-ts` v0.31.0 and `@graphprotocol/graph-cli` v0.51.0 the `loadRelated` method is available.
+
+This enables loading derived entity fields from within an event handler. For example, given the following schema:
+
+```graphql
+type Token @entity {
+  id: ID!
+  holder: Holder!
+  color: String
+}
+
+type Holder @entity {
+  id: ID!
+  tokens: [Token!]! @derivedFrom(field: "holder")
+}
+```
+
+The following code will load the `Token` entity that the `Holder` entity was derived from:
+
+```typescript
+let holder = Holder.load('test-id')
+// Load the Token entity that the Holder entity was derived from
+let token = holder.tokens.load()
+```
+
 #### Updating existing entities
 
 There are two ways to update an existing entity: