[docs] Add reference to install_db4.sh in OS X build instructions

Author: jamesob

doc/build-osx.md

@@ -26,6 +26,20 @@ If you want to build the disk image with `make deploy` (.dmg / optional), you ne
 
 NOTE: Building with Qt4 is still supported, however, could result in a broken UI. Building with Qt5 is recommended.
 
+Berkeley DB
+-----------
+It is recommended to use Berkeley DB 4.8. If you have to build it yourself,
+you can use [the installation script included in contrib/](contrib/install_db4.sh)
+like so
+
+```shell
+./contrib/install_db4.sh .
+```
+
+from the root of the repository.
+
+**Note**: You only need Berkeley DB if the wallet is enabled (see the section *Disable-Wallet mode* below).
+
 Build Bitcoin Core
 ------------------------