Add a new readme section for IRB extension #2329

Job	Run time
ruby-versions / Generate Ruby versions	2s
rake test 2.7 (latest reline)	41s
rake test 2.7	42s
rake test 3.0 (latest reline)	28s
rake test 3.0	37s
rake test 3.1 (latest reline)	41s
rake test 3.1	27s
rake test 3.2 (latest reline)	31s
rake test 3.2	28s
rake test head (latest reline)	30s
rake test head	34s
rake test truffleruby-head (latest reline)	6m 27s
rake test truffleruby-head	5m 11s
vterm-yamatanooroti 2.7 (latest reline)	1m 11s
vterm-yamatanooroti 2.7	1m 27s
vterm-yamatanooroti 3.0 (latest reline)	39s
vterm-yamatanooroti 3.0	52s
vterm-yamatanooroti 3.1 (latest reline)	57s
vterm-yamatanooroti 3.1	50s
vterm-yamatanooroti 3.2 (latest reline)	43s
vterm-yamatanooroti 3.2	49s
vterm-yamatanooroti head (latest reline)	46s
vterm-yamatanooroti head	43s
	26m 16s

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