Add link to emacs password store.

contrib/related-projects.txt

@@ -9,3 +9,5 @@ The following projects make use of pass:
 * passmenu: A demnu script.
   <https://github.com/cdown/passmenu>
 
+* emacs-password-store: Emacs package
+  <https://github.com/svend/emacs-password-store>