[RFC] Explain how to extract svds-factors

[dkarrasch]

Fixes JuliaLang/julia#18. I have shamelessly taken the original JuliaLang-PR as a basis for further modifications. Some changes:

• I have removed the claim that U*Diagonal(S)*Vt yield A, and reformulated towards low-rank approximation.
• I'm not sure the sentence

Internally Vt is stored and hence Vt is more efficient to extract than V.

is true anymore, since Vt is just wrapped by an Adjoint. I kept the sentence for the time being.

• In the doctest, I removed the (redundant) reference to sparsity to avoid additional package dependency, in case that's an issue.

[coveralls]

Pull Request Test Coverage Report for Build 97

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.279%

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Totals
Change from base Build 96:	0.0%
Covered Lines:	183
Relevant Lines:	272

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #52 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      JuliaLang/julia#52   +/-   ##
=======================================
  Coverage   67.27%   67.27%           
=======================================
  Files           2        2           
  Lines         272      272           
=======================================
  Hits          183      183           
  Misses         89       89

Impacted Files	Coverage Δ
src/Arpack.jl	50.35% <ø> (ø)	⬆️

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