documentation: Validating nested arrays after creation with class-transformer

[Sauermann]

Description

In the README.md section about validating arrays containing nested objects, the requirements for validating objects created with class-transformer are not made clear. This issue regards the following section of the documentation:

class-validator/README.md

Lines 332 to 342 in 9f0c3e9

	It also works with multi-dimensional array, like :
	```typescript
	import { ValidateNested } from 'class-validator';
	export class Plan2D {
	@ValidateNested()
	matrix: Point[][];
	}
	```

The problem is, that after creating a nested array with class-transformer with the above code as template, the objects within the array do not contain the class information.

Proposed solution

I suggest to include the following additional information:

When validating nested objects created with class-transformer, it is required to specify the type of the nested objects as described in the class-transformer documentation.

[vlapo]

I agree we can add small note about this information.

[gilsoncav]

I also agree. Stumbled here with the same misleading assumption. It worths a note.