Add info about exported types to README.md

Author: flaksp

README.md

@@ -19,3 +19,13 @@ Import `JsonSchema` type definition:
 ```typescript
 import { JsonSchema } from '@fosfad/json-schema-typescript-definitions/2020-12';
 ```
+
+## Documentation
+
+### Exported types
+
+The package exports 3 types: `JsonSchemaBoolean`, `JsonSchemaObject` and `JsonSchema`:
+
+- `JsonSchemaBoolean` stands for [Boolean JSON Schema](https://json-schema.org/draft/2020-12/json-schema-core.html#rfc.section.4.3.2);
+- `JsonSchemaObject` is an object with all possible JSON Schema keywords within;
+- `JsonSchema` is a union type between `JsonSchemaBoolean` and `JsonSchemaObject`.