add sources

Author: jrising

content/getting-started.md

@@ -11,6 +11,7 @@ development. Here are some of the relationships that have been
 uncovered in recent years:
 
 ![Examples from Carleton and Hsiang (2016).](images/examples.jpg)
+Source: [Carleton and Hsiang (2016)](https://science.sciencemag.org/content/353/6304/aad9837).
 
 This tutorial will walk you through the steps necessary to relate
 socioeconomic outcomes to weather data at high resolution. We will

content/reduced-form-specification.md

@@ -106,6 +106,7 @@ $$F(T_{it})=\sum_{k\in \{1,2,...,K\}} \beta^k*f(T_{it}^k)$$
 
 **Example of reduced-form regression plots for different functional forms**
 ![Data from  Greenstone et al. (2019)!](images/fform_cil.JPG)
+Source: [Carleton et al. (2019)](https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3224365).
 
 ## 2.3. Cross-validation
 

content/weighting-schemes.md

@@ -262,6 +262,7 @@ will be the addition of a map projection. You'll want to choose your
 projection carefully, since people are bound to judge you for it.
 
 ![Considerations for projections.](https://imgs.xkcd.com/comics/map_projections.png)
+Source: [XKCD 977](https://www.xkcd.com/977/).
 
 Using the projection, we can now make the final version of this
 figure. Note that you will need to use `geom_tile` rather than