Proof out post-extra and pre-req

DESIGN.md

@@ -31,11 +31,31 @@ flexibility. So, advanced application packagers can leverage Ansible if they
 need it and for simple applications or pre-packaged applications, the work will
 already be done.
 
+## Pre-requisites
+
+On the Dokku host.
+
+```bash
+$ dokku plugin:install https://github.com/decentral1se/dokku-ansible-deploy.git
+$ dokku plugin:install-dependencies
+$ dokku ansible-deploy:vault-password $APP  # add per-app vault password for secrets decrypting
+```
+
+In the application repository (still to be implemented).
+
+```bash
+$ echo "TODO"
+```
+
 ## Triggers
 
 ### post-extract
 
-TODO.
+1. Check if there is a `deploy.d` folder in the application repository
+1. If not, just output a message about skipping and stop execution
+1. If so, create an internal deploy folder (e.g. `$DOKKU_LIB_ROOT/data/deploy.d/$APP`)
+1. Copy over all application deploy files to the internal folder (e.g. `cp -r "$TMPDIR/deploy.d/." "$DOKKU_LIB_ROOT/data/deploy.d/$APP"`)
+1. Generate a vault password file and set correct permissions for protection
 
 ### pre-deploy