Add details about BAC

.catalog/README.md

@@ -9,6 +9,10 @@
 
 (If it works for yours, submit a PR to add your country flag)
 
+## To use:
+
+eMTRD are secured to prevent people from reading the data on a passport just by bumping into it.  The data is secured using a key based on the passport number, date of birth, and date of expiry.  A real passport machine reads these values from the MRZ (Machine Readable Zone, the ones with ">") using a camera. For the app, you have to enter the values manually.  The app will then generate the key and read the data using a system called BAC(https://en.wikipedia.org/wiki/Basic_access_control).
+
 ## Notes:
  - Caches MRZ info to make subsequent use faster
 

README.md

@@ -10,8 +10,9 @@
 
 (If it works for yours, submit a PR to add your country flag)
 
-## Notes:
- - Caches MRZ info to make subsequent use faster
+## To use:
+
+eMTRD are secured to prevent people from reading the data on a passport just by bumping into it.  The data is secured using a key based on the passport number, date of birth, and date of expiry.  A real passport machine reads these values from the MRZ (Machine Readable Zone, the ones with ">") using a camera. For the app, you have to enter the values manually.  The app will then generate the key and read the data using a system called BAC(https://en.wikipedia.org/wiki/Basic_access_control).
 
 ## Limitations
  - Does not parse some of the optional DG (under 'advanced' menu)