[readme] add section on bare `nvm install` #1112

Job	Run time
shellcheck_matrix (bash, nvm.sh)	49s
shellcheck_matrix (sh, nvm.sh)	47s
shellcheck_matrix (dash, nvm.sh)	1m 10s
shellcheck_matrix (ksh, nvm.sh)	59s
shellcheck_matrix (bash, install.sh)	39s
shellcheck_matrix (bash, bash_completion)	50s
shellcheck_matrix (bash, nvm-exec)	40s
shellcheck	11s
	6m 5s

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