keychain: Add long description

Copied from the project home page today.

pkgs/tools/misc/keychain/default.nix

@@ -38,5 +38,17 @@ stdenv.mkDerivation rec {
       with stdenv.lib;
       platforms.linux ++ platforms.darwin;
     maintainers = with stdenv.lib.maintainers; [ sigma ];
+    longDescription = ''
+			Keychain helps you to manage SSH and GPG keys in a convenient and secure
+			manner. It acts as a frontend to ssh-agent and ssh-add, but allows you
+			to easily have one long running ssh-agent process per system, rather
+			than the norm of one ssh-agent per login session.
+
+			This dramatically reduces the number of times you need to enter your
+			passphrase. With keychain, you only need to enter a passphrase once
+			every time your local machine is rebooted. Keychain also makes it easy
+			for remote cron jobs to securely "hook in" to a long-running ssh-agent
+			process, allowing your scripts to take advantage of key-based logins.
+    '';
   };
 }