TBB build dependency not documented

[MLopez-Ibanez]

Building pagmo2 now requires TBB or one gets the error:

-- Looking for TBB...
CMake Error at /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find TBB (missing: TBB_INCLUDE_DIR tbb)
Call Stack (most recent call first):
  /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
  cmake_modules/FindTBB.cmake:513 (find_package_handle_standard_args)
  CMakeLists.txt:106 (find_package)

The solution in Ubuntu is:

sudo apt install libtbb-dev

This should be documented in https://esa.github.io/pagmo2/install.html#

[bluescarni]

Already reported in #285, we'll update the installation instructions in the upcoming release.

[bluescarni]

Addressed in #340.

[bluescarni]

Fixed.