Installation instructions for FastStringSearch

For Translate API:messagegroups it improves
performance quite a bit, tested with ab:

              min  mean[+/-sd] median   max
Total:       6790 7126 451.5   6985    8532
Total (fss): 2253 2354  72.7   2353    2547

Change-Id: I110f58d598930c9eb2e795f9e0e8a65e0f8599ff