Add privacy policy

PRIVACYPOLICY.MD

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+# BlueDot Privacy Policy
+
+The BlueDot Python library and Android app do not store your personal data. That is it!
+
+There is nowhere to store your data, no database, no server. Even if Rick Astley asked [really really nicely](https://www.youtube.com/watch?v=dQw4w9WgXcQ) to see your data it couldn't be given it to him.
+
+For the purposes of utmost transparency...  The android app will store 2 items of non-personal data, [preferences for using the default port and a preferred port](https://github.com/martinohanlon/BlueDot/blob/master/clients/android/app/src/main/java/com/stuffaboutcode/bluedot/SettingsActivity.java) (if you change them from the default) in a [shared preferences file](https://developer.android.com/training/data-storage/shared-preferences).