add build badge to readme #15

Job	Run time
License headers check	6s
Soundness / Broken symlinks check	2s
Soundness / API breakage check	2m 20s
Soundness / Documentation check	42s
Soundness / License headers check	0s
Soundness / Unacceptable language check	3s
Soundness / Format check	44s
Soundness / YAML lint check	6s
Soundness / Python lint check	0s
Soundness / Shell check	0s
Swift 6 Language Mode / Swift 6 language mode	1m 27s
Unit tests / Unit tests / Linux (5.9)	23s
Unit tests / Unit tests / Linux (5.10)	1m 24s
Unit tests / Unit tests / Linux (6.0)	1m 46s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 0s
Unit tests / Unit tests / Linux (nightly-main)	2m 6s
	13m 9s

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