Clarify extension creation instructions in README #209

Job	Run time
test-ubuntu-ruby (ruby-head, 11-3.0)	45s
test-ubuntu-ruby (ruby-head, 12-master)	55s
test-ubuntu-ruby (ruby-head, 13-master)	1m 0s
test-ubuntu-ruby (ruby-head, 14-master)	45s
test-ubuntu-ruby (ruby-head, 15-master)	45s
test-ubuntu-ruby (3.2, 11-3.0)	38s
test-ubuntu-ruby (3.2, 12-master)	48s
test-ubuntu-ruby (3.2, 13-master)	42s
test-ubuntu-ruby (3.2, 14-master)	56s
test-ubuntu-ruby (3.2, 15-master)	46s
test-ubuntu-ruby (3.1, 11-3.0)	1m 0s
test-ubuntu-ruby (3.1, 12-master)	1m 47s
test-ubuntu-ruby (3.1, 13-master)	1m 14s
test-ubuntu-ruby (3.1, 14-master)	56s
test-ubuntu-ruby (3.1, 15-master)	58s
test-ubuntu-ruby (3.0, 11-3.0)	1m 0s
test-ubuntu-ruby (3.0, 12-master)	1m 0s
test-ubuntu-ruby (3.0, 13-master)	55s
test-ubuntu-ruby (3.0, 14-master)	1m 5s
test-ubuntu-ruby (3.0, 15-master)	2m 3s
	19m 58s

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