README.md in lbfgsb-0.1.0 vs README.md in lbfgsb-0.2.0

@@ -1,9 +1,10 @@
 # Lbfgsb.rb
 
 [![Build Status](https://github.com/yoshoku/lbfgsb.rb/workflows/build/badge.svg)](https://github.com/yoshoku/lbfgsb.rb/actions?query=workflow%3Abuild)
 [![Gem Version](https://badge.fury.io/rb/lbfgsb.svg)](https://badge.fury.io/rb/lbfgsb)
 [![BSD 3-Clause License](https://img.shields.io/badge/License-BSD%203--Clause-orange.svg)](https://github.com/yoshoku/suika/blob/master/LICENSE.txt)
+[![Documentation](http://img.shields.io/badge/api-reference-blue.svg)](https://yoshoku.github.io/lbfgsb.rb/doc/)
 
 Lbfgsb.rb is a Ruby binding for [L-BFGS-B](http://users.iems.northwestern.edu/~nocedal/lbfgsb.html)
 that is a limited-memory algorithm for solving large nonlinear optimization problems
 subject to simple bounds on the variables.
 L-BFGS-B is written in FORTRAN. Author converted the codes into C-lang