Add links for world cup

Author: jayb

index.csv

@@ -1,4 +1,5 @@
 dataset_url,article_url,live
+https://github.com/fivethirtyeight/data/tree/master/world-cup-2018,https://projects.fivethirtyeight.com/2018-world-cup-predictions/,TRUE
 https://github.com/fivethirtyeight/data/tree/master/special-elections,https://fivethirtyeight.com/features/be-skeptical-of-anyone-who-tells-you-they-know-how-democrats-can-win-in-november/,
 https://github.com/fivethirtyeight/data/tree/master/congress-trump-score,https://projects.fivethirtyeight.com/congress-trump-score/,TRUE
 https://github.com/fivethirtyeight/data/tree/master/reluctant-trump,https://fivethirtyeight.com/features/reluctant-trump-voters-swung-the-election-heres-how-they-think-hes-doing/,

world-cup-2018/README.md

@@ -0,0 +1,12 @@
+---
+files:
+  - https://projects.fivethirtyeight.com/soccer-api/international/2018/wc_matches.csv
+  - https://projects.fivethirtyeight.com/soccer-api/international/2018/wc_forecasts.csv
+---
+# World Cup 2018
+
+This file contains links to the data behind our [2018 World Cup Predictions](https://projects.fivethirtyeight.com/2018-world-cup-predictions/).
+
+`wc_matches.csv` contains match-by-match projections.
+
+`wc_forecasts.csv` contains our tournament forecasts.