Add description of “Macro-economic targets” in policies.rst

As discussed [here](https://github.com/iiasa/message_doc/pull/25#discussion_r332522693) in #25.

Open policies.rst and read the discussion linked from pull request #25 for the intended content of the “Macro-economic targets” section. Add the description in the appropriate location, then verify that the section clearly documents the target and matches the surrounding policy documentation.

Written by the indexing model from the issue text.