Add contributing instructions to README.md

[mrandri19]

No description provided.

[codecov]

Codecov Report

Merging #154 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   89.25%   89.16%   -0.09%     
==========================================
  Files          14       14              
  Lines        1135     1135              
==========================================
- Hits         1013     1012       -1     
- Misses        122      123       +1     

Impacted Files	Coverage Δ
src/Optimization.jl	88.38% <0.00%> (-0.18%)	⬇️

---

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 07addeb...9b1b0f2. Read the comment docs.