docs: README Contributing — mention PR merge comment on linked issues #320

Job	Run time
Coverage (llvm-cov)	2m 27s
security	25s
cargo-deny (advisories + licenses)	4m 2s
mutation	1m 43s
version	5s
lint	1m 20s
test (windows-latest)	46s
test (ubuntu-latest)	40s
test (macos-latest)	38s
	12m 6s

Provide feedback