Add Custom Validation Message as a rails til

rails/custom-validation-message.md

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+# Custom Validation Message
+
+When using Rails validations, a standard error message will be provided
+whenever there is a violation. Consider the scenario when there is a
+uniqueness validation on the email attribute and it is violated:
+
+```ruby
+# User model
+validates_uniqueness_of :email
+
+# Users controller
+new_user.errors.full_messages
+#=> ["Email has already been taken"]
+```
+
+Sometimes you don't want the default validation message. The validation
+declaration can be given a `message` option to specify an alternate
+validation message.
+
+```ruby
+# User model
+validates_uniqueness_of :email, message: 'is not available'
+
+# Users controller
+new_user.errors.full_messages
+#=> ["Email is not available"]
+```
+
+Keep in mind that `full_messages` will prepend the model name to the front
+of the message. You'll want to ensure that the resulting message is
+coherent.