Add PRIVACY.md

PRIVACY.md

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+# Privacy Policy
+
+This application doesn't store by itself any data anywhere other than your phone. No tracking or ads are included and
+no user information is collected, sent, or processed in any central database. Configuration information is stored
+locally, on your phone.
+
+## Data sent to Indico Servers
+
+Once you pair the application with an Indico event, you will be establishing a link with the server where it lives.
+Once you start checking in users, the application may share the following basic information with the target server:
+ * The event's unique ID;
+ * The user's unique Registration ID at the event in question;
+ * A unique, random, secret code generated along with the ticket which is used to prove its authenticity;
+ * The user's "checked-in" status ("yes" or "no").
+
+If you would like to know how this data is collected and processed on the target server, please refer to its own
+Privacy Policy.