Added free alternative to Google Authenticator (FreeOTP).

doc/workflow/two_factor_authentication.md

@@ -21,7 +21,8 @@ is to know your username and password *and* have access to your phone.
 
 **On your phone:**
 
-1. Install a compatible application. We recommend [Google Authenticator].
+1. Install a compatible application. We recommend [Google Authenticator]
+(properitary) or [FreeOTP] (FLOSS).
 1. In the application, add a new entry in one of two ways:
     * Scan the code with your phone's camera to add the entry automatically.
     * Enter the details provided to add the entry manually.
@@ -63,3 +64,4 @@ your phone's application or a recovery code to log in.
 1. Click **Disable Two-factor Authentication**.
 
 [Google Authenticator]: https://support.google.com/accounts/answer/1066447?hl=en
+[FreeOTP]: https://fedorahosted.org/freeotp/
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