[doc] Mention `.document` file in README #830

Job	Run time
lint	15s
ruby-versions / Generate Ruby versions	7s
test (3.0, ubuntu-latest)	28s
test (3.0, macos-latest)	25s
test (3.0, windows-latest)	1m 52s
test (3.1, ubuntu-latest)	30s
test (3.1, macos-latest)	24s
test (3.1, windows-latest)	2m 18s
test (3.2, ubuntu-latest)	32s
test (3.2, macos-latest)	23s
test (3.2, windows-latest)	2m 26s
test (3.3, ubuntu-latest)	27s
test (3.3, macos-latest)	26s
test (3.3, windows-latest)	3m 6s
test (head, ubuntu-latest)	40s
test (head, macos-latest)	27s
test (head, windows-latest)	1m 38s
test (jruby, ubuntu-latest)	2m 16s
test (jruby, macos-latest)	1m 30s
test (jruby-head, ubuntu-latest)	2m 0s
test (jruby-head, macos-latest)	1m 30s
test (truffleruby, ubuntu-latest)	2m 26s
test (truffleruby, macos-latest)	1m 43s
test (truffleruby-head, ubuntu-latest)	3m 10s
test (truffleruby-head, macos-latest)	2m 7s
	33m 6s

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