Add README at top of repository with links to others.

Test: looked at README in Markdown preview.
Change-Id: I254f9ea81d6dbc0267f2d2d3b36f779a55dc03cd

README.md

+# Virtualization
+
+This repository contains userspace services related to running virtual machines on Android,
+especially protected virtual machines. See the
+[getting started documentation](docs/getting_started/index.md) and
+[Microdroid README](microdroid/README.md) for more information.