[long_run_growth] Motivation for `long_run_growth` lecture

[HengchengZhang]

• Explain the real GDP/income
• we need to discuss if and when GDP is important
  -- what is it correlated with? health? happiness?
  -- is more GDP better for rich countries?
• mention alternative measures to GDP
• the first lecture needs to explain from the start what the point of
  the lecture is (policy matters)