docs: explain what the OS-X package does and does not provide.

docs/OS-X.rst

@@ -0,0 +1,23 @@
+==============
+OS-X Packaging
+==============
+
+Pre-built Tahoe-LAFS ".pkg" installers for OS-X are generated with each
+source-code commit. These installers offer an easy way to get Tahoe and all
+its dependencies installed on your Mac. They do not yet provide a
+double-clickable application: after installation, you will have a "tahoe"
+command-line tool, which you can use from a shell (a Terminal window) just as
+if you'd installed from source.
+
+Installers are available from this directory:
+
+ https://tahoe-lafs.org/source/tahoe-lafs/tarballs/OS-X-packages/
+
+Download the latest .pkg file to your computer and double-click on it. This
+will install to /Applications/tahoe.app, however the app icon there is not
+how you use Tahoe (launching it will get you a dialog box with a reminder to
+use Terminal). `/Applications/tahoe.app/bin/tahoe` is the executable. You can
+symlink this to /usr/local/bin/ or ~/bin/ to make it more convenient to run.
+
+Tahoe-LAFS can also be installed on OS-X with pip, as described in the
+README.