Add protobuf to build.sh

- build protobuf from build script
- added a note in readme to install protobuf, forestdb before
  installing secondary indexes

Change-Id: If4dc88ba3f9053431fbac7233e19eeaa9ee3d3dc
Reviewed-on: http://review.couchbase.org/39079
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>