[DOC] New document "For Users" (#856)

Author: BurdetteLamar

doc/echo.rb

@@ -0,0 +1,13 @@
+require 'reline'
+
+puts 'Welcome to the Echo program!'
+puts '  To exit, type Ctrl-d in empty line.'
+
+# Words for completion.
+Words = %w[ foo_foo foo_bar foo_baz qux ]
+Reline.completion_proc = proc { |word| Words }
+
+# REPL (Read-Evaluate-Print Loop).
+while line = Reline.readline(prompt = 'echo> ', history = true)
+  puts "You typed: '#{line.chomp}'."
+end