📝 docs: add data doc (#33)

Co-authored-by: Mateusz Gostański <[email protected]>

Author: grzywaczek

docs/data.md

@@ -0,0 +1,17 @@
+## Identifier - unique identifier specified by the user, providing its key from TSchema or a function resolving it.
+
+The `ResolveIdentifier` type is intended to specify the type of identifier in the Schema uniquely.
+
+If the identifier has not been specified by the user, the DefaultIdentifier will be returned as a fallback to `string`.
+
+If specified, it must be a key in the Schema or a function generating the identifier. In the case of being a key, the returned identifier will be of the corresponding value type.
+
+If it is a function, the type will match the type returned from the function.
+
+| Identifier | Resolved type      |
+|------------|--------------------|
+| `undefined`| `string`           |
+| `keyof Schema` | `Schema[key]`    |
+| `(data: Schema) => Foo` | `Foo`       |
+
+The UpdateEntityInput type is used in functions updating data and aims to secure against updating the identifier to ensure data integrity.