Update README with spotlight development slack channel #1292

Job	Run time
build	6h 0m 3s
build	5m 5s
test (ruby 3.1 / rails 7.0.8 / blacklight latest )	11m 16s
test (ruby 3.1 / rails 7.0.8 / blacklight latest )	10m 58s
test (ruby 3.2 / rails 7.0.8 / blacklight latest )	11m 5s
test (ruby 3.2 / rails 7.0.8 / blacklight latest )	11m 27s
test (ruby 3.0 / rails 6.1.7.6 / blacklight 7.33.1 )	11m 13s
test (ruby 3.0 / rails 6.1.7.6 / blacklight 7.33.1 )	11m 13s
test (ruby 2.7 / rails 6.1.7.6 / blacklight 7.33.1 )	11m 17s
test (ruby 2.7 / rails 6.1.7.6 / blacklight 7.33.1 )	11m 28s
test (ruby 3.1 / rails 7.0.8 / blacklight 8.0.0 )	11m 17s
test (ruby 3.1 / rails 7.0.8 / blacklight 8.0.0 )	11m 29s
test (ruby 3.2 / rails 7.0.8 / blacklight 8.0.0 / Propshaft)	11m 0s
test (ruby 3.2 / rails 7.0.8 / blacklight 8.0.0 / Propshaft)	11m 11s
	8h 20m 2s

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