Update README.md

Adds information about the new `add_singleton!` function for `DisjointSets`.
Changed name from add_singleton! to push!
Merge pull request #25 from dpsanders/MakeSet

Change `add_singleton!` (was `make_set!`) in `disjoint_set.jl` to `push!`
Avoid stack overflow in length method. Use iterator in show method
Allow iteration on empty list
Add linked list to module
Added linked list to readme
Inserted missing comma
Implemented tests for list.jl
Implemented filter and reverse functions for list