add public goods

concepts/public-goods.md

@@ -1 +1,21 @@
-# Public goods
+# Public Goods
+
+Public goods are goods which anyone can use and benefit from, not just those who contribute to the creation and/or upkeep of the good.
+
+* Public goods are non-excludable: anyone can use them.  
+* Public goods are non-rival: one person's consumption of the good does not diminish another person's consumption of it.
+
+| Rival         | Non-Excludable |      Excludable       |
+| -----------   | ------------   | ----------------------|
+| Yes           | Private goods  | Common-Pool Resources |
+| No            | Club goods     | Public Goods          |
+
+## Examples of public goods:
+
+* National defence
+* Open source software 
+* City parks
+* Lighthouses
+* Street lights
+
+See [Public Goods Problem](public-goods-problem.md)