Add section about the new MeiliDocumentContainer<T> class in readme

[ahmednfwela]

this PR #350 added MeiliDocumentContainer<T> but its usage is optional.
we need to document to users how to use it, and use it more in the code samples.

to opt-in for it, users need to call mapToContainer() extension methods on search results.

relevant test:

meilisearch-dart/test/search_test.dart

Lines 471 to 500 in d110365

	test('vector search', () async {
	final vector = [0, 1, 2];
	final res = await index
	.search(
	null,
	SearchQuery(
	vector: vector,
	),
	)
	.asSearchResult()
	.mapToContainer();
	expect(res.vector, vector);
	expect(
	res.hits,
	everyElement(
	isA<MeiliDocumentContainer<Map<String, dynamic>>>()
	.having(
	(p0) => p0.vectors,
	'vectors',
	isNotNull,
	)
	.having(
	(p0) => p0.semanticScore,
	'semanticScore',
	isNotNull,
	),
	),
	);
	});