doc: mention creating application support bitcoin folder on OSX

Author: Jack Mallers

Diff for: doc/build-osx.md

@@ -82,6 +82,8 @@ Bitcoin Core is now available at `./src/bitcoind`
 
 Before running, you may create an empty configuration file:
 
+    mkdir -p "/Users/${USER}/Library/Application Support/Bitcoin"
+
     touch "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf"
 
     chmod 600 "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf"