doc: flesh out the README

README.md

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-# content
+# owid-content
+
+_Data explorers used on the Our World In Data site._
+
+## Overview
+
+Data explorers are configurable collections of Grapher charts, rendered as a single embeddable iframe. Explorers are configured using a spreadsheet-like format, expressed in TSV files. The TSV files can be updated manually, or more commonly using the grapher admin interface. Large explorers with many combinations may have their configs generated by scripts instead.
+
+Like all aspects of the Our World In Data site, data explorers are statically generated at deploy time by code in the [owid-grapher](https://github.com/owid/owid-grapher) codebase.
+
+For example the [Covid-19 Data Explorer](https://ourworldindata.org/explorers/coronavirus-data-explorer) is generated by the file `explorers/coronavirus-data-explorer.explorer.tsv`.