Clarify macos install in INSTALL.md.

Make the binary package installer the recommended method, and
note that homebrew installs from source and takes a lot of
disk space.

Closes #4661.

INSTALL.md

@@ -17,9 +17,6 @@
 
 ## macOS
 
-  - You can install pandoc using
-    [homebrew](http://brew.sh): `brew install pandoc`.
-
   - There is a package installer at pandoc's [download page].
     If you later want to uninstall the package, you can do so
     by downloading [this script][uninstaller]
@@ -30,6 +27,12 @@
     unzip the file and move the binaries and man pages to
     whatever directory you like.
 
+  - Alternatively, you can install pandoc using
+    [homebrew](http://brew.sh): `brew install pandoc`.
+    Note: This method installs pandoc from source, so it
+    will take a long time and a lot of disk space for the
+    ghc compiler and dependent Haskell libraries.
+
   - For PDF output, you'll also need LaTeX.  Because a full [MacTeX]
     installation takes more than a gigabyte of disk space, we recommend
     installing [BasicTeX](http://www.tug.org/mactex/morepackages.html)