Add table of contents to README

[MohanSha]

add table of contents

[codecov]

Codecov Report

Merging #435 into master will decrease coverage by 6.96%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #435      +/-   ##
=========================================
- Coverage   78.46%   71.5%   -6.97%     
=========================================
  Files          25      25              
  Lines        1416    1400      -16     
=========================================
- Hits         1111    1001     -110     
- Misses        305     399      +94

Impacted Files	Coverage Δ
src/poisson_solvers.jl	40.94% <0%> (-56.72%)	⬇️
src/fields.jl	49.35% <0%> (-15.59%)	⬇️
src/Oceananigans.jl	62.5% <0%> (-12.5%)	⬇️
src/utils.jl	78.26% <0%> (-7.32%)	⬇️
src/time_steppers.jl	72.02% <0%> (-2.1%)	⬇️
src/boundary_conditions.jl	85.24% <0%> (-1.64%)	⬇️
src/output_writers.jl	79.54% <0%> (-0.57%)	⬇️
src/models.jl	82.05% <0%> (+1.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d32cf20...d88b63f. Read the comment docs.

[ali-ramadhan]

Hi @MohanSha thank you for this PR! Didn't know you could have a table of contents in the README. This should be useful especially as we add more stuff to the README.

I just have a couple of suggestions:

1. "Table of contents" feels a little formal to me, I might change the name to just "Contents".
2. I might also remove * [Oceananigans.jl](#oceananigansjl) as the top-level item in the list. I think it makes more sense to have "Installation instructions", "Running your first model", etc. to be top-level items.

[ali-ramadhan]

Woops, sorry I closed the PR by mistake but reopened it.

[ali-ramadhan]

Hmmm, I also think the movie hyperlinks in the "Contents" should link to sections in the README instead of to YouTube directly.

I guess that would be

[Deep convection](#deep-convection)

instead of

[<a href="https://www.youtube.com/watch?v=kpUrxnKKMjI" rel="nofollow">Deep convection</a>](#deep-convection)

[ali-ramadhan]

Thank you for the revisions @MohanSha! Looks great!