Dotfiles template default repo (#224)

Co-authored-by: Muhammad Atif Ali <me@matifali.dev>

dotfiles/README.md

@@ -18,3 +18,16 @@ module "dotfiles" {
   agent_id = coder_agent.example.id
 }
 ```
+
+## Setting a default dotfiles repository
+
+You can set a default dotfiles repository for all users by setting the `default_dotfiles_repo` variable:
+
+```tf
+module "dotfiles" {
+  source                = "registry.coder.com/modules/dotfiles/coder"
+  version               = "1.0.12"
+  agent_id              = coder_agent.example.id
+  default_dotfiles_repo = "https://github.com/coder/dotfiles"
+}
+```