Add documentation

[amontoison]

#149

[coveralls]

Coverage decreased (-0.2%) to 96.123% when pulling 3734281 on amontoison:docs into db180a0 on JuliaSmoothOptimizers:master.

[codecov]

Codecov Report

Merging #152 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #152   +/-   ##
======================================
  Coverage    96.9%   96.9%           
======================================
  Files          24      24           
  Lines        1972    1972           
======================================
  Hits         1911    1911           
  Misses         61      61

Impacted Files	Coverage Δ
src/Krylov.jl	100% <ø> (ø)	⬆️
src/cg_lanczos.jl	100% <ø> (ø)	⬆️
src/krylov_utils.jl	100% <ø> (ø)	⬆️

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[amontoison]

Ok, thanks for the review. I also changed some indentations and add a comment about the AbstractLinearOperator required for A.
If you want to see what the documentation looks like before merging the pull request: build.zip

[dpo]

Thank you!