Teach watchman to ignore buck-out directory

This is useful to ignore a directory that contains only build products
and where file change notifications are unwanted because of the sheer
volume of files.  Not to mention that during tests execution temporary
directories are created and wiped out that may confuse watchman.

This file is provided in the same way as buckversion and must be linked
to the main Buck project directory to have effect, e. g.:

  $ cd <project_dir>
  $ ln -s ../bucklets/watchmanconfig .watchmanconfig

Change-Id: I61ad9f768cce0fec505854008c5f6118f967accc
diff --git a/watchmanconfig b/watchmanconfig
new file mode 100644
index 0000000..8d9aeea
--- /dev/null
+++ b/watchmanconfig
@@ -0,0 +1,5 @@
+{
+  "ignore_dirs": [
+    "buck-out"
+  ]
+}