Add note about E2E tests in `README` #290

Job	Run time
lint	34s
define-matrix	5s
tests (kusama/src/assetHubKusama.karura.test.ts)	1m 5s
tests (kusama/src/basilisk.karura.test.ts)	1m 14s
tests (kusama/src/coretimeKusama.kusama.test.ts)	44s
tests (kusama/src/karura.kusama.test.ts)	52s
tests (kusama/src/karura.shiden.test.ts)	1m 0s
tests (kusama/src/kusama.shiden.test.ts)	53s
tests (kusama/src/kusamaGovernance.e2e.test.ts)	1m 26s
tests (kusama/src/peopleKusama.e2e.test.ts)	2m 4s
tests (kusama/src/peopleKusama.kusama.test.ts)	43s
tests (polkadot/src/acala.assetHubPolkadot.test.ts)	46s
tests (polkadot/src/acala.astar.test.ts)	1m 30s
tests (polkadot/src/acala.moonbeam.test.ts)	2m 43s
tests (polkadot/src/acala.polkadot.test.ts)	53s
tests (polkadot/src/assetHubPolkadot.polkadot.test.ts)	34s
tests (polkadot/src/astar.polkadot.test.ts)	1m 14s
tests (polkadot/src/coretimePolkadot.polkadot.test.ts)	40s
tests (polkadot/src/hydration.moonbeam.test.ts)	2m 21s
tests (polkadot/src/peoplePolkadot.e2e.test.ts)	1m 22s
tests (polkadot/src/peoplePolkadot.polkadot.test.ts)	1m 4s
tests (polkadot/src/polkadotGovernance.e2e.test.ts)	1m 7s
all-passed	1s
	24m 55s

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