Update readme

README

@@ -1,14 +1,48 @@
-How To
-======
+Why/What
+========
 
-0. power off device
-1. insert source USB in the TOP usb slot
-2. insert target USB in the BOTTOM usb slot
-3. wait. wait some more. it's slow and can take 30-60 minutes depending on how
-   many document conversions take place
-4. when the only status LED left is the power indicator on the rPI, the process
-   is finished
-5. power off the device and disconnect the drives
+This project aims to be used in case you got an USB key you do not know what is
+contains but still want to have a look.
+
+Work in progress, contributions welcome:
+
+The content of the first key will be copyed or/and converted to the second key
+following theses rules (based on the mime type):
+- direct copy of plain text files (mime type: text/*)
+- direct copy of audio files (mime type: audio/*)
+- direct copy of image files (mime type: image/*)
+- direct copy of video files (mime type: video/*)
+- direct copy of example files (mime type: example/*)
+- direct copy of message files (mime type: message/*)
+- direct copy of model files (mime type: model/*)
+- direct copy of multipart files (mime type: multipart/*)
+- Copying or converting the application files this way (mime type: application/*):
+  - pdf => HTML
+  - msword|vnd.openxmlformats-officedocument.*|vnd.ms-*|vnd.oasis.opendocument* => pdf => html
+  - *xml* => copy as a text file
+  - x-dosexec (executable) => prepend and append DANGEROUS to the filename
+  - x-gzip|x-tar|x-7z-compressed => compressed file
+  - octet-stream => direct copy
+
+Compressed files (x-gzip|x-tar|x-7z-compressed):
+- Unpacking of archives
+- Recursively run the rules on the unpacked files
+
+Usage
+=====
+
+0. Power off the device
+1. Plug the untrusted key in the top usb slot of the Raspberry Pi
+2. Plug your own key in the bottom usb slot
+   Note: This key should be bigger than the original one because the archives
+   will be copyed
+3. Optional: connect the HDMI cable to a screen to see what happen
+4. Connect the power to the micro USB
+5. Wait until you do not see any blinking green light on the board, or if you
+   connected the HDMI cable, check the screen
+   it's slow and can take 30-60 minutes depending on how many document
+   conversions take place
+6. Power off the device and disconnect the drives
 
 Notes
 =====