Readme: `index by` is supported #147

Job	Run time
Platform test (7.4)	2m 57s
Platform test (8.0)	3m 14s
Platform test (8.1)	3m 5s
Platform test (8.2)	3m 17s
Platform test (8.3)	3m 2s
Platform test (8.4)	2m 59s
Platform test (8.1, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 57s
Platform test (8.2, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	3m 0s
Platform test (8.3, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 28s
Platform test (8.4, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 38s
	29m 37s

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