Add project-context.md for spanner-to-sourcedb #6620

Job	Run time
Build	1m 21s
Checkstyle	2m 10s
Spotless	2m 23s
Unit Tests	10m 13s
Dataflow Templates Integration Smoke Tests	0s
Dataflow Templates Integration Tests	-1s
Dataflow Templates Load Tests	-1s
	16m 5s

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