added contribution guidelines

[lucaferranti]

PR description

Added contribution guidelines, containing a quick git 101 and a few coding and documentation guidelines.

Other

As you can probably deduce by the work of this PR, I was pretty bored today.

[codecov-commenter]

Codecov Report

Merging #81 (1bcae5f) into main (f4951eb) will decrease coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
- Coverage   97.08%   96.42%   -0.66%     
==========================================
  Files          12       12              
  Lines         309      336      +27     
==========================================
+ Hits          300      324      +24     
- Misses          9       12       +3     

Impacted Files	Coverage Δ
src/linear_systems/enclosures.jl	85.33% <0.00%> (-2.55%)	⬇️
src/rref.jl	100.00% <0.00%> (ø)
src/classify.jl	100.00% <0.00%> (ø)
src/multiplication.jl	100.00% <0.00%> (ø)
src/linear_systems/verify.jl	100.00% <0.00%> (ø)
src/linear_systems/oettli_linear.jl	100.00% <0.00%> (ø)
src/linear_systems/oettli_nonlinear.jl	100.00% <0.00%> (ø)
src/utils.jl	97.67% <0.00%> (+0.11%)	⬆️

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