[doc] Mention `.document` file in README #831

Job	Run time
lint	19s
ruby-versions / Generate Ruby versions	2s
test (3.0, ubuntu-latest)	26s
test (3.0, macos-latest)	22s
test (3.0, windows-latest)	1m 44s
test (3.1, ubuntu-latest)	31s
test (3.1, macos-latest)	22s
test (3.1, windows-latest)	1m 34s
test (3.2, ubuntu-latest)	26s
test (3.2, macos-latest)	20s
test (3.2, windows-latest)	1m 50s
test (3.3, ubuntu-latest)	28s
test (3.3, macos-latest)	29s
test (3.3, windows-latest)	2m 53s
test (head, ubuntu-latest)	32s
test (head, macos-latest)	20s
test (head, windows-latest)	1m 31s
test (jruby, ubuntu-latest)	2m 4s
test (jruby, macos-latest)	1m 32s
test (jruby-head, ubuntu-latest)	1m 58s
test (jruby-head, macos-latest)	1m 11s
test (truffleruby, ubuntu-latest)	2m 19s
test (truffleruby, macos-latest)	2m 10s
test (truffleruby-head, ubuntu-latest)	3m 16s
test (truffleruby-head, macos-latest)	2m 8s
	30m 47s

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