[JOSS Review] Installing SleepECG with optional dependencies

[richrobe]

(as part of JOSS Review)

When installing SleepECG with extras, following the instructions in the Contributing guide dev dependencies fails when using zsh as shell (which is now the default unser ) because square brackets need to be escaped. Thus, I would maybe suggest to add an additional note that zsh users should call pip install -e ".[dev]" (with quotes). This command should also work when using other shells (such as bash).

[cbrnr]

Thanks, I've added quotes in all instances where this was mentioned.