README.md in lbfgsb-0.5.0 vs README.md in lbfgsb-0.5.1

@@ -1,13 +1,13 @@
 # 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/main/LICENSE.txt)
-[![Documentation](http://img.shields.io/badge/api-reference-blue.svg)](https://yoshoku.github.io/lbfgsb.rb/doc/)
+[![Documentation](https://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)
+Lbfgsb.rb is a Ruby binding for [L-BFGS-B](https://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
 and call that with Ruby native exntesion.
 
@@ -24,9 +24,23 @@
     $ bundle install
 
 Or install it yourself as:
 
     $ gem install lbfgsb
+
+Notes: lbfgsb.rb uses 32-bit integer for the integer type in its native code.
+If you want to use 64-bit integer, give the installation option as below:
+
+```
+$ gem install lbfgsb -- --with-use-int64
+```
+
+In adition, if you want to use an external BLAS library for linear algebra on LBFGSB optimization,
+give the directory and library in the installation options as follows:
+
+```
+$ gem install lbfgsb -- --with-blas-dir=/opt/local/openblas/lib --with-blas-lib=openblas
+```
 
 ## Usage
 Example 1. Logistic Regression
 
 ```ruby