update docs on data transfer

docs/PLAYBOOKS.md

@@ -7,6 +7,7 @@ There are five main playbooks :
 - ssh_setup.yml # set up ssh keys
 - start_sens_sniffer.yml
 - stop_sens_sniffer.yml
+- data_transfer.yml # retrieve results
 - test_meta.yml # test functions
 ```
 ## First master-nodes connection: SSH setup
@@ -72,6 +73,12 @@ If they are, we set up a crontab to run the sniffer when planned, and a crontab
 **`stop_sens_sniffer.yml`**
 This playbook interrupts any running `sniffer.py`, and cancels planned crontabs.
 
+## Retrieving experiment data from nodes
+`data_transfer.yml` finds all .pcap and .gps files from our target folder for each nonde
+- and copies them inside ../results/ssX/sY/filename
+[TODO]: transfer to meta-server
+[TODO]: delete og files when success
+
 ## Testing devices and statuses
 `test_meta.yml` runs all tests playbooks.
 [TODO] detailler les tests