Add details to the docs fixing #8 and #9

[gdalle]

fix #8 and #9 by

• specifying that self-loops are not supported
• specifying that constructors from edge lists should avoid duplicate edges

[codecov]

Codecov Report

Merging #42 (344b7ee) into master (d995ffd) will not change coverage.
The diff coverage is n/a.

❗ Current head 344b7ee differs from pull request most recent head 8788527. Consider uploading reports for the commit 8788527 to get more accurate results

@@           Coverage Diff           @@
##           master      #42   +/-   ##
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  Coverage   94.83%   94.83%           
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  Files           9        9           
  Lines         445      445           
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  Hits          422      422           
  Misses         23       23           

Impacted Files	Coverage Δ
src/SimpleWeightedGraphs.jl	100.00% <ø> (ø)
src/simpleweighteddigraph.jl	96.47% <ø> (ø)
src/simpleweightedgraph.jl	93.39% <ø> (ø)

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[simonschoelly]

Since when are self-loops not supported? There might be some issues, but apart from, we have always tried to support them. I don't think we can simply add to the documentation that they are now not supported.