Add prerequisites to README

It's better to be open about these upfront to make the installation process smoother.

README.md

@@ -1,6 +1,12 @@
 # puppet-vault
 Puppet module for managing Hashicorp's [Vault](https://vaultproject.io/)
 
+##Prerequisites
+This module requires:
+
+- [puppet-staging](https://github.com/nanliu/puppet-staging)
+- a `zip` package to be installed (eg `package { "zip": ensure => installed }`)
+
 ##Installation
 
 To simply install the vault binary:
@@ -34,4 +40,4 @@ Open an [issue](https://github.com/EvanKrall/puppet-vault/issues) or
 
 ##Acknowledgements
 
-Much of the initial code was adapted from [solarkennedy/puppet-consul](https://github.com/solarkennedy/puppet-consul). Thank you to the contributors to that project.
+Much of the initial code was adapted from [solarkennedy/puppet-consul](https://github.com/solarkennedy/puppet-consul). Thank you to the contributors to that project.