Add some thoughts on security.

Signed-off-by: Reimar Döffinger <[email protected]>

README.md

@@ -3,6 +3,31 @@ qtpass
 
 QtPass is a gui for [pass](http://www.passwordstore.org/)
 
+Security considerations
+-----------------------
+Using this program will not magically keep your passwords secure against
+compromised computers even if you use it in combination with a smartcard.
+It does protect future and changed passwords though against anyone with access to
+your password store only but not your keys.
+Used with a smartcard it also protects against anyone just monitoring/copying
+all files/keystrokes on that machine and such an attacker would only gain access
+to the passwords you actually use.
+Once you plug in your smartcard and enter your PIN all your passwords available
+to the machine can be decrypted by it, if there is malicious software targeted
+specifically against it installed (or at least one that knows how to use a
+smartcard).
+To get better protection out of use with a smartcard even against a targeted
+attack I can think of at least two options:
+* The smartcard must require explicit confirmation for each decryption operation.
+  Or if it just provides a counter for decrypted data you could at least notice
+  an attack afterwards, though at quite some effort on your part.
+* Use a different smartcard for each (group of) key.
+* If using a YubiKey or U2F module or similar that requires a "button" press for
+  other authentication methods you can use one OTP/U2F enabled WebDAV account per
+  password (or groups of passwords) as a quite inconvenient workaround.
+  Unfortunately I do not know of any WebDAV service with OTP support except ownCloud
+  (so you would have to run your own server).
+
 Current state
 -------------
 * Using pass or directly with git and gpg2