Add links to the documentation in README.md

Author: Erwan Thomas

README.md

@@ -1,3 +1,11 @@
 Sheetah is a library designed to process tabular data from any kind (CSV
 documents, XSLX sheets, etc...) with a focus on typed results and a
 fine-grain error management.
+
+## Documentation
+
+The following resources document the latest valid state of the `master`
+branch:
+
+- [Ruby API](https://steeple-org.github.io/sheetah/ruby)
+- [Coverage](https://steeple-org.github.io/sheetah/coverage)