Add Fail as a ruby til

ruby/fail.md

@@ -0,0 +1,22 @@
+# Fail
+
+The `fail` method is synonymous with `raise` in Ruby. Consider the following
+code snippet:
+
+```ruby
+def is_odd?(num)
+  num.odd?
+rescue
+  fail StandardError, 'you cannot use odd on this'
+end
+
+> is_odd?(1)
+#=> true
+> is_odd?('hello')
+#=> StandardError: you cannot use odd on this
+```
+
+Nevertheless, I believe the `raise` method is preferred to the `fail`
+method.
+
+[source](http://ruby-doc.org/core-2.3.0/Kernel.html#method-i-fail)