docs: add 2.4.0 release notes

Author: sgratzl

content/covidcast/release-log/headless/v2.4.0.md

@@ -0,0 +1,15 @@
+---
+title: v2.4.0
+date: 2021-05-03
+---
+
+### Signal Changes
+ * Remove "At Away Location 6hr+" and "At Away Location 3-6hr" since the underlying data is not longer available
+ * Add "In-Person Schooling" to survey dashboard
+
+### New Features
+ * add hint about data freshness
+ * add warnings about common missing data
+
+
+More information: https://github.com/cmu-delphi/www-covidcast/releases/tag/v2.4.0