[README] Add a CI badge for the main branch #95

Job	Run time
Compatibility test	4m 56s
Swift 6 Language Mode	0s
Integration test / Linux (5.8)	22s
Example packages / Linux (5.8)	21s
Integration test / Linux (5.9)	1m 39s
Soundness / API breakage check	2m 36s
Example packages / Linux (5.9)	16m 37s
Integration test / Linux (5.10)	1m 38s
Example packages / Linux (5.10)	15m 5s
Integration test / Linux (nightly-6.0)	2m 0s
Example packages / Linux (nightly-6.0)	21m 40s
Integration test / Linux (nightly-main)	28s
Soundness / Broken symlinks check	3s
Example packages / Linux (nightly-main)	28s
Soundness / Documentation check	1m 40s
Soundness / Unacceptable language check	4s
Soundness / Format check	39s
Soundness / License headers check	5s
Soundness / Shell check	28s
Unit tests / Unit tests / Linux (5.8)	24s
Unit tests / Unit tests / Linux (5.9)	1m 31s
Unit tests / Unit tests / Linux (5.10)	1m 41s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 9s
Unit tests / Unit tests / Linux (nightly-main)	28s
	1h 17m 2s

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