Add polling averages (#41)

* add data for historical polling averages to 'polls' dataset

* rename historical polling averages file

* remove link to 2020 current polling avgs

* remove blank columns from CSV

* add link to 2020 Presidential Primary Polling Averages

polls/README.md

@@ -7,11 +7,13 @@ files:
   - https://projects.fivethirtyeight.com/polls-page/governor_polls.csv
   - https://projects.fivethirtyeight.com/polls-page/president_approval_polls.csv
   - https://projects.fivethirtyeight.com/polls-page/generic_ballot_polls.csv
+  - https://projects.fivethirtyeight.com/2020-primary-data/pres_primary_avgs_2020.csv
 ---
 
-# Polls
 
-This file contains links to the data behind [Latest Polls](https://projects.fivethirtyeight.com/polls/).
+This file contains links to the polling data behind our [Latest Polls](https://projects.fivethirtyeight.com/polls/) dashboard.
+
+## Polls
 
 - [Presidential Primary Polls](https://projects.fivethirtyeight.com/polls-page/president_primary_polls.csv)
 - [Presidential General Election Polls](https://projects.fivethirtyeight.com/polls-page/president_polls.csv)
@@ -21,4 +23,9 @@ This file contains links to the data behind [Latest Polls](https://projects.five
 - [Presidential Approval Polls](https://projects.fivethirtyeight.com/polls-page/president_approval_polls.csv)
 - [Generic Ballot Polls](https://projects.fivethirtyeight.com/polls-page/generic_ballot_polls.csv)
 
+## Polling Averages
 
+- [Presidential Primary Polling Averages (2020)](https://projects.fivethirtyeight.com/2020-primary-data/pres_primary_avgs_2020.csv)
+- [Presidential Primary Polling Averages (1980-2016)](./pres_primary_avgs_1980-2016.csv)
+
+Please see [What Makes Our New 2020 Democratic Primary Polling Averages Different](https://fivethirtyeight.com/features/what-makes-our-new-2020-democratic-primary-polling-averages-different/) for more information about the methodology used to calculate polling averages.