Update README.md

Author: sbromberger

README.md

@@ -5,3 +5,16 @@
 [![Coverage Status](https://coveralls.io/repos/JuliaGraphs/GraphIO.jl/badge.svg?branch=master&service=github)](https://coveralls.io/github/JuliaGraphs/GraphIO.jl?branch=master)
 
 [![codecov.io](http://codecov.io/github/JuliaGraphs/GraphIO.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaGraphs/GraphIO.jl?branch=master)
+
+GraphIO provides support to [LightGraphs.jl](https://github.com/JuliaGraphs/LightGraphs.jl) for reading/writing graphs in various formats.
+
+Currently, the following functionality is provided:
+
+Format | Read | Write
+-------|------|-----
+GML | ✓ | ✓
+Graph6 | ✓ | ✓
+GraphML | ✓ | ✓
+Pajek NET | ✓ | ✓
+GEXF | | ✓
+DOT | ✓ |