Merge #15802: doc: mention creating application support bitcoin folde…

…r on OSX

8dfbb5c doc: mention creating application support bitcoin folder on OSX (Jack Mallers)

Pull request description:

  When setting up bitcoin core on my new MacBook via the [`macOS Build Instructions and Notes`](https://github.com/bitcoin/bitcoin/blob/master/doc/build-osx.md), running `touch "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf"` resulted in `No such file or directory` because my `/Users/${USER}/Library/Application Support/Bitcoin/` folder had not been created yet.

  This PR adds `mkdir "/Users/${USER}/Library/Application Support/Bitcoin"` to the documentation before creating the configuration file with `touch "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf"`

ACKs for commit 8dfbb5:
  fanquake:
    utACK 8dfbb5c

Tree-SHA512: c401dae0a681bed77b31939bd29f79837f66c8537da393871fc3596e8c10629b8817ece9c8d49b1b9de34e74e0fbff742a9116b02ce7a71af93f872b87539337

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"