docs: Embellish README
Add some brief explanatory text and a few pointers.
Fixes
-# documentation
-Kata Containers documentation
+# Documentation
+
+The [Kata Containers](https://github.com/kata-containers)
+documentation repository hosts overall system documentation, with information
+common to multiple components.
+
+For details of the other Kata Containers repositories, see the
+[repository summary](https://github.com/kata-containers/kata-containers).
