Add an initial README

(Per opentracing/opentracing.io#144)

README.md

@@ -1,2 +1,9 @@
-# specification
-A place to document (and discuss) the OpenTracing specification
+# The OpenTracing Specification repository
+
+The github.com/opentracing/specification repository is a place to document (and discuss) the OpenTracing specification itself (independent of any particular language or platform).
+
+Historically this activity took place on the docs repo ((github.com/opentracing/opentracing.io)[https://github.com/opentracing/opentracing.io]), but that became awkward and the `specification` repository was proposed as part of [opentracing.io#144](https://github.com/opentracing/opentracing.io/issues/144).
+
+## Current Status
+
+This repository is just getting off the ground. In the future it will house some `.yaml` files (or their moral equivalents) that document various constants shared across repos, and it will definitely be the place to discuss larger OT matters via Github Issues.