Buck: Document Watchman bug related to symbolic links

Buck has currently one critical bug when Buck daemon is used in
combination with symbolic links: [1]. Document the workaround not
to use Buck daemon in this scenario.

[1] https://github.com/facebook/buck/issues/341

Change-Id: I7cdc19c7245b499e71196d16aab0785dda187fa3
1 file changed