Add documentation for custom commands, accounts restriction.

docs/user-guide/custom-commands.md

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+# Custom Commands
+
+One powerful feature of Moonshot is the ability to add custom
+project-specific commands. You can hook into many of the features
+provided by the `Moonshot::Controller` and `Moonshot::Stack`
+interface, and make native calls to AWS using the AWS SDK for Ruby
+v2.
+
+To do this, place a file in `moonshot/cli_extensions/my_command.rb`
+that extends `Moonshot::Command` (or `Moonshot::SSHCommand`, if you
+want to use the Moonshot SSH interface).
+
+## Example
+
+```ruby
+# moonshot/cli_extensions/get_elb_address.rb
+class GetElbAddress < Moonshot::Command
+  self.description = 'Display the ELBs external hostname.'
+  self.usage       = 'get-elb-address'
+
+  def execute
+    puts controller.stack.outputs['APIElasticLoadBalancerDNS']
+  end
+end
+```
+
+## Account Restrictions
+
+If you have a development- or production-only command you want
+restricted to running in appropriately labelled accounts, you can use
+the `only_in_account` option. This can be either a single string, or
+an array of strings. For example:
+
+```ruby
+# moonshot/cli_extensions/drop_database.rb
+class DropDatabase < Moonshot::Command
+  self.description     = 'Delete all the data (development only!)'
+  self.usage           = 'drop-database'
+  self.only_in_account = 'company-dev'
+
+  def execute
+    # ...
+  end
+end
+```
+
+If the IAM account alias for the current AWS account does not exactly
+match, the user will receive an error:
+
+```
+'drop-database' can only be run in the following accounts:
+- company-dev
+```