Added Ubuntu Linux Instructions

[yaaqov]

I've linked to a TeX Stackexchange article which has a straightforward answer on installing Xelatex on Ubuntu Linux (essential for saving Jupyter notebooks as PDFs).

[mpacer]

I think it’d be better to just mention the content directly; though it shouldn’t need sudo(I think?).

[takluyver]

apt commands do indeed need sudo, because they act systemwide. Without sudo, you can usually only modify your home directory, /tmp and $XDG_RUNTIME_DIR.

[takluyver]

But since the answer is basically one command, I'd agree that we can inline it rather than linking to stackexchange.

[mpacer]

@yaaqov Could you change this to give the instructions directly rather than linking to the stack exchange answer?

[mpacer]

I’m going to push this change into this branch in a bit.

[takluyver]

Pushed the change, merging now. Thanks!