Improve the bash completion script installation instructions.

Summary:
On some distributions there is a specific folder in which the script should
be placed, rather than adding the `source` command in the user profile.

https://github.com/facebook/buck/pull/56

Test Plan: Follow the instructions and make sure it works as expected.

scripts/bash_completion

diff --git a/scripts/bash_completion b/scripts/bash_completion
index af84124..631d6f2 100644
--- a/scripts/bash_completion
+++ b/scripts/bash_completion
@@ -17,7 +17,12 @@
 # Installing
 # ==========
 #
-# To install the completion, point to this file from your .bash_profile, like so:
+# To install the completion, copy this file to the appropriate folder for
+# your distribution, for example:
+#
+#     cp ~/path/to/bash_completion /etc/bash_completion.d/buck
+#
+# Alternatively you can invoke this file from your .bash_profile, like so:
 #
 #     . ~/path/to/bash_completion
 #