Add pkg-config to Ubuntu Installation instructions

[jonahweissman]

I followed the directions on the README and got the following error when running cmake:

-- Could NOT find PkgConfig (missing:  PKG_CONFIG_EXECUTABLE) 
CMake Error at /usr/share/cmake-3.5/Modules/FindPkgConfig.cmake:418 (message):
  pkg-config tool not found
Call Stack (most recent call first):
  /usr/share/cmake-3.5/Modules/FindPkgConfig.cmake:532 (_pkg_check_modules_internal)
  depends/libsnark/depends/libfqfft/CMakeLists.txt:92 (pkg_check_modules)

I installed pkg-config and was able to successfully run cmake.

This PR adds pkg-config to the list of packages that should be installed for Ubuntu 16.04 LTS, so others don't have to install pkg-config seperately. It might also make sense to include pkg-config for 14.04, as well.