readme: add note on trialing specific shells #298

Job	Run time
homebrew-macos-test	3m 19s
login-shells-and-linting	2m 43s
distro-test (ubuntu:latest)	2m 39s
distro-test (fedora:latest)	3m 13s
distro-test (debian:latest)	2m 36s
distro-test (alpine:latest)	2m 27s
distro-test (manjarolinux/base)	2m 44s
distro-test (opensuse/leap)	3m 12s
distro-test (opensuse/tumbleweed)	3m 17s
distro-test (kalilinux/kali-rolling)	3m 10s
distro-test (voidlinux/voidlinux)	2m 51s
fresh-macos-test	3m 20s
	35m 31s

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