Document the usage of the RateSampler

Author: LotharSee

docs/index.rst

@@ -96,6 +96,24 @@ API
 
 .. _integrations:
 
+
+Sampling
+--------
+
+It is possible to sample traces with `ddtrace`.
+While the Trace Agent already samples traces to reduce the bandwidth usage, this client sampling
+reduces performance overhead.
+
+`RateSampler` samples a ratio of the traces. Its usage is simple::
+
+    from ddtrace.sampler import RateSampler
+
+    # Sample rate is between 0 (nothing sampled) to 1 (everything sampled).
+    # Sample 50% of the traces.
+    sample_rate = 0.5
+    tracer.sampler = RateSampler(sample_rate)
+
+
 Integrations
 ------------