Add a readme for stats pointing to the top-level headers. (#122)

Author: Ian Sturdy

opencensus/stats/README.md

@@ -0,0 +1,15 @@
+# Stats API
+
+The primary stats API documentation is in the headers in `opencensus/stats`.
+See the following classes and headers for the top-level interfaces:
+
+### Recording data
+- A [`Measure`](measure.h) specifyies the resources against which data is
+  recorded.
+- [`recording.h`](recording.h) defines the recording function.
+
+### Accessing data
+- A [`ViewDescriptor`](view_descriptor.h) defines what data a view collects,
+  and provides an interface for registering it for export.
+- A [`View`](view.h) provides a handle for accessing data for a view within the
+  task.