update keyserver and provide fingerprint

content/install.txt

@@ -40,7 +40,10 @@ The lines prefixed with ``sudo`` have to be run as root.
 
 2. (Optional) Add the OpenPGP key used to sign the repositories by executing::
 
-       sudo apt-key adv --keyserver pgp.mit.edu --recv-key C3F147F5980F3535
+       sudo apt-key adv --keyserver pool.sks-keyservers.net --recv-key C3F147F5980F3535
+
+   The fingerprint of the key is 5229 9057 FAD7 9965 3C4F 088A C3F1 47F5 980F
+   3535.
 
 3. Update the package data, and install Kwant::