add doxygen dependency into README

[yslan]

This is me being dumb.
I forget to install doxygen, but the README should tell user do so.

[aprilnovak]

Yes, good point! Unless I made a mistake, the PR into nekRS doesn't have any doxygen stuff - when I said how to build these docs, I meant build them in the nekRS repository itself, not in this repo. Where are you building the documentation exactly?

[yslan]

I clone this repo and install it with make html.
Will try the NekRS/doc one.

[yslan]

NekRS/doc does work without doxygen. Will start from there.
Thanks

[aprilnovak]

For the sake of reviewing the nekRS PR, please build the documentation over on the nekRS repo - it's a little different than what's shown here and actually represents what will be merged.

Thanks!