Create RELEASE.md

RELEASE.md

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+Release type: minor
+
+The Query Codegen system now supports the `@oneOf` directive.
+
+When writing plugins, you can now access `GraphQLObjectType.is_one_of` to determine if the object being worked with has a `@oneOf` directive.
+
+The default plugins have been updated to take advantage of the new attribute.
+
+For example, given this schema:
+
+```python
+@strawberry.input(one_of=True)
+class OneOfInput:
+    a: Optional[str] = strawberry.UNSET
+    b: Optional[str] = strawberry.UNSET
+
+
+@strawberry.type
+class Query:
+    @strawberry.field
+    def one_of(self, value: OneOfInput) -> str: ...
+
+
+schema = strawberry.Schema(Query)
+```
+
+And this query:
+
+```graphql
+query OneOfTest($value: OneOfInput!) {
+  oneOf(value: $value)
+}
+```
+
+The query codegen can now generate this Typescript file:
+
+```typescript
+type OneOfTestResult = {
+    one_of: string
+}
+
+type OneOfInput = {
+    a: string,
+    b: never
+} | {
+    a: never,
+    b: string
+}
+
+type OneOfTestVariables = {
+    value: OneOfInput
+}
+```