message: add Apache 2.0 licencse note
diff: @@ -31,6 +31,7 @@ CmdStan's source-code repository is hosted here on GitHub.
  ### Licensing
  The core Stan C++ code and CmdStan are licensed under new BSD.
  +Note that the Stan math library depends on the Intel TBB library which is licensed under the Apache 2.0 license. This dependency implies an additional restriction as compared to the new BSD lincense alone. The Apache 2.0 license is incompatible with GPL-2 licensed code if distributed as a unitary binary. You may refer to the Apache 2.0 evaluation page on the [Stan Math wiki](https://github.com/stan-dev/math/wiki/Apache-2.0-License-Evaluation).
  ## Installation
  1. Download the latest release tarball (use the "green" link) from: [CmdStan releases](https://github.com/stan-dev/cmdstan/releases)
  