Readme: `index by` is supported #146

Job	Run time
Platform test (7.4)	3m 14s
Platform test (8.0)	3m 2s
Platform test (8.1)	3m 17s
Platform test (8.2)	3m 36s
Platform test (8.3)	3m 34s
Platform test (8.4)	3m 22s
Platform test (8.1, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	2m 50s
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...	3m 0s
Platform test (8.4, doctrine/orm:^3.0 doctrine/dbal:^4.0 carbonphp/carbon-doctrine-types:^3 gedmo...	3m 14s
	32m 9s

Provide feedback