Update build information

Add the command necessary to add ppa repository in Ubuntu

Refs #3757

doc/build-unix.md

@@ -65,6 +65,10 @@ for Ubuntu 12.04 and later:
 	sudo apt-get install libboost-all-dev
 
  db4.8 packages are available [here](https://launchpad.net/~bitcoin/+archive/bitcoin).
+ You can add the repository using the following command:
+
+        sudo add-apt-repository ppa:bitcoin/bitcoin
+        sudo apt-get update
 
  Ubuntu 12.04 and later have packages for libdb5.1-dev and libdb5.1++-dev,
  but using these will break binary wallet compatibility, and is not recommended.