Update developer installation instructions

[cdvv7788]

Make changes as required at #862

[cdvv7788]

@jhamrick Is this what you had in mind for #862 ?

[jhamrick]

Thanks!

I think it would be a bit clearer for new developers that all three commands should be run if the they are one right after another with no linebreaks/comments, e.g.:

jupyter nbextension install --symlink --sys-prefix --py nbgrader
jupyter nbextension enable --sys-prefix --py nbgrader
jupyter serverextension enable --sys-prefix --py nbgrader

The other documentation (e.g. about what the symlink is doing) could go before or after (but ideally not as comments in the code block itself).

[jhamrick]

(See my comment above)

[cdvv7788]

@jhamrick Better?

[jhamrick]

Looks great now, thanks!