Documents need for ruby-dev/ruby-devel

Related issue:

$ sudo gem install jekyll
[sudo] password for dhiana:
Building native extensions.  This could take a while...
ERROR:  Error installing jekyll:
	ERROR: Failed to build gem native extension.

    /usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /usr/share/include/ruby.h