Update README to include point release instructions #2691

Job	Run time
Extract tasks from matrix	4s
check-migrations (polkadot, polkadot-runtime, relay/polkadot, wss://try-runtime.polkadot.io:443, ...	44m 45s
check-migrations (kusama, staging-kusama-runtime, relay/kusama, wss://try-runtime-kusama.polkadot...	31m 15s
check-migrations (asset-hub-kusama, asset-hub-kusama-runtime, system-parachains/asset-hubs/asset-...	31m 47s
check-migrations (asset-hub-polkadot, asset-hub-polkadot-runtime, system-parachains/asset-hubs/as...	1h 40m 32s
check-migrations (bridge-hub-kusama, bridge-hub-kusama-runtime, system-parachains/bridge-hubs/bri...	24m 16s
check-migrations (bridge-hub-polkadot, bridge-hub-polkadot-runtime, system-parachains/bridge-hubs...	40m 19s
check-migrations (collectives-polkadot, collectives-polkadot-runtime, system-parachains/collectiv...	24m 50s
check-migrations (coretime-kusama, coretime-kusama-runtime, system-parachains/coretime/coretime-k...	24m 9s
check-migrations (coretime-polkadot, coretime-polkadot-runtime, system-parachains/coretime/coreti...	23m 39s
check-migrations (people-kusama, people-kusama-runtime, system-parachains/people/people-kusama, w...	24m 24s
check-migrations (people-polkadot, people-polkadot-runtime, system-parachains/people/people-polka...	24m 37s
check-migrations (encointer-kusama, encointer-kusama-runtime, system-parachains/encointer, wss://...	23m 42s
All migrations passed	1s
	6h 58m 20s

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