[DOC] New document "For Users"

doc/echo.rb

@@ -0,0 +1,21 @@
+require 'reline'
+require 'open-uri'
+
+# Get words for completion.
+words_url = 'https://raw.githubusercontent.com/first20hours/google-10000-english/refs/heads/master/google-10000-english-usa-no-swears-long.txt'
+words = []
+URI.open(words_url) do |file|
+  while !file.eof?
+    words.push(file.readline.chomp)
+  end
+end
+# Install completion proc.
+Reline.completion_proc = proc { |word|
+  words
+}
+puts 'Welcome to the Echo program!'
+puts '  To exit, type Ctrl-d in empty line.'
+# REPL (Read-Evaluate-Print Loop)
+while line = Reline.readline(prompt = 'echo> ', history = true)
+  puts "You typed: '#{line.chomp}'."
+end