Update install-flyctl.html.markerb

[anadon]

Added bash and bash-like shell specific modifications required to directly invoke Fly.io commands. Input taken from https://stackoverflow.com/a/30387689/826573 for prepending to text files.

For very new people, this kind of thing can trip them up.

Summary of changes

Related Fly.io community and GitHub links

n/a if none

Notes

One odd part is prepending to .bashrc. This is to not conflict with direnv.