update Arch installation instructions

content/install.txt

@@ -85,8 +85,17 @@ documentation will be installed locally in the directory
 Arch Linux
 ==========
 
-`Arch install scripts for Kwant <https://aur.archlinux.org/packages/python2-kwant/>`_ are kindly provided by Max Schlemmer.
-To install, follow the `Arch User Repository installation instructions <https://wiki.archlinux.org/index.php/Arch_User_Repository#Installing_packages>`_.
+Arch install scripts for Kwant are kindly provided by Jörg Behrmann (formerly
+by Max Schlemmer).  To install, follow the `Arch User Repository installation
+instructions
+<https://wiki.archlinux.org/index.php/Arch_User_Repository#Installing_packages>`_.
+Note that for checking the validity of the package you need to add the key
+used for signing to your user's keyring via::
+
+    gpg --keyserver pool.sks-keyservers.net --recv-key C3F147F5980F3535
+
+The fingerprint of the key is 5229 9057 FAD7 9965 3C4F 088A C3F1 47F5 980F
+3535.
 
 
 Mac OS X