Add details to the docs fixing #8 and #9 (#42)

fix #8 and #9 by
- specifying that self-loops are not supported
- specifying that constructors from edge lists should avoid duplicate
edges

Author: gdalle

docs/src/index.md

@@ -20,6 +20,8 @@ Because `SimpleWeighted(Di)Graph`s are stored in sparse matrices, they have two
 
 - Zero-weight edges are discarded by `add_edge!`. A possible workaround is to [set a very small weight instead](https://stackoverflow.com/questions/48977068/how-to-add-free-edge-to-graph-in-lightgraphs-julia/48994712#48994712).
 
+In additions, self-loops are not supported.
+
 ## Alternatives
 
 If your graphs have more than just edge weights to store, take a look at [MetaGraphsNext.jl](https://github.com/JuliaGraphs/MetaGraphsNext.jl) or [MetaGraphs.jl](https://github.com/JuliaGraphs/MetaGraphs.jl) for more complex formats.

src/SimpleWeightedGraphs.jl

@@ -1,7 +1,7 @@
 """
     SimpleWeightedGraphs
 
-A package for graphs with edge weights, stored as sparse adjacency matrices.
+A package for graphs with edge weights and no self-loops, stored as sparse adjacency matrices.
 """
 module SimpleWeightedGraphs
 

src/simpleweighteddigraph.jl

@@ -18,6 +18,7 @@ SimpleWeightedDiGraph(adjmx; permute)  # from adjacency matrix, possibly transpo
 SimpleWeightedDiGraph(sources, destinations, weights)  # from list of edges
 ```
 Use `methods(SimpleWeightedDiGraph)` for the full list of constructors.
+When building a new graph from a list of edges, be aware that repeating `(src, dst)` pairs may lead to undefined behavior (e.g. due to floating point errors during weight addition).
 """
 mutable struct SimpleWeightedDiGraph{T<:Integer,U<:Real} <: AbstractSimpleWeightedGraph{T,U}
     weights::SparseMatrixCSC{U,T}

src/simpleweightedgraph.jl

@@ -19,6 +19,7 @@ SimpleWeightedGraph(adjmx)  # from adjacency matrix
 SimpleWeightedGraph(sources, destinations, weights)  # from list of edges
 ```
 Use `methods(SimpleWeightedGraph)` for the full list of constructors.
+When building a new graph from a list of edges, be aware that repeating `(src, dst)` pairs may lead to undefined behavior (e.g. due to floating point errors during weight addition).
 """
 mutable struct SimpleWeightedGraph{T<:Integer,U<:Real} <: AbstractSimpleWeightedGraph{T,U}
     weights::SparseMatrixCSC{U,T}