Add Listing Local Variables as a ruby til.

ruby/listing-local-variables.md

@@ -0,0 +1,30 @@
+# Listing Local Variables
+
+In Ruby 2.2, the `binding` object gives us access to a method
+`#local_variables` which returns the symbol names of the binding's local
+variables.
+
+```ruby
+def square(x)
+  puts binding.local_variables.inspect
+  x.times do |a|
+    puts binding.local_variables.inspect
+  end
+  z = x * x
+  puts binding.local_variables.inspect
+  z
+end
+square(2)
+```
+
+which results in
+
+```ruby
+[:x, :z]
+[:a, :x, :z]
+[:a, :x, :z]
+[:x, :z]
+=> 4
+```
+
+[source](http://ruby-doc.org/core-2.2.0/Binding.html#method-i-local_variables)