add velocypack to source or install instructions

[ObiWahn]

velocypack needs to be installed on the target system it should be added to the repository or mentioned in the install instructions.

[ObiWahn]

If it does not come with the source it is important to be able to configure the find_package with some custom path best via environment variable (e.g. VELOCYPACK_DIR=$USER/.local/velocypack) so the package can be compiled without super user powers and without knowledge of CMake. This might be especially important for windows as something as a default library directories are not as well defined as on UNIX like systems.