added main page on docs

Author: matbesancon

docs/src/index.md

@@ -15,3 +15,11 @@ Pages = ["LightGraphsFlows.jl"]
 Order = [:function, :type]
 ```
 
+This is the documentation page for `LightGraphsFlows`. In all pages, we assume
+LightGraphsFlows has been imported into scope and that LightGraphs is 
+available with the alias `lg`:
+```julia
+using LightGraphsFlows
+import LightGraphs
+const lg = LightGraphs
+```