other endpoints data source intro

Author: nmdefries

docs/api/epidata_signals.md

@@ -0,0 +1,23 @@
+---
+title: Data Sources and Signals
+parent: Other Endpoints (COVID-19 and Other Diseases)
+nav_order: 2
+has_children: true
+
+---
+
+# Delphi's Epidata Data Sources and Signals
+
+Delphi's Epidata endpoints include the following data sources.
+Most of these sources are updated weekly.
+Unlike [the main endpoint](covidcast.md), the format of different Epidata endpoints varies.
+The API for retrieving data from these sources is described in the
+[API clients documentation](client_libraries.md).
+
+To obtain many of these signals and update them daily, Delphi has written
+extensive software to obtain data from various sources, aggregate the data,
+calculate statistical estimates, and format the data to be shared through the Delphi Epidata API.
+This code is
+[open source and available on GitHub](https://github.com/cmu-delphi/delphi-epidata),
+and contributions are welcome.
+