add readme to sdk-experimental (#1451)

* add readme to sdk-experimental

* Update sdk_experimental/README.md

Co-authored-by: Francis Bogsanyi <[email protected]>

---------

Co-authored-by: Francis Bogsanyi <[email protected]>

Author: plantfansam

sdk_experimental/README.md

@@ -0,0 +1,8 @@
+# Experimental SDK
+
+The experimental SDK houses bleeding-edge(-ish) functionality that may have unstable APIs. Some examples of what's in
+here:
+
+* `traceresponse` propagator (in [w3c `trace-context` editor's draft](https://w3c.github.io/trace-context/))
+* Consistent Probability Sampler (marked as `experimental` in [opentelemetry-specification)](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/tracestate-probability-sampling.md))
+