Add String Interpolation With Instance Variables as a ruby til

ruby/string-interpolation-with-instance-variables.md

@@ -0,0 +1,31 @@
+# String Interpolation With Instance Variables
+
+When using regular variables with string interpolation in Ruby, they must be
+wrapped in curly braces (e.g. `"This is a #{variable}"`). With instance
+variables (and class and global variables) you can just use the _octothorp_
+followed directly by the variable.
+
+Here is an example of this in action:
+
+```ruby
+class Person
+  def initialize(name)
+    @name = name
+  end
+
+  def whoami
+    puts "I am #@name"
+  end
+end
+
+bob = Person.new("bob")
+#=> #<Person:0x007fdaf3291618 @name="bob">
+
+bob.whoami
+# I am bob
+```
+
+This is a handy shortcut, but may affect readability and/or result in an
+interpolation error at some point. Your mileage may vary.
+
+h/t Josh Davey