docs: Update README with details on SDK builder overwriting issue (#351)

## Which problem is this PR solving?
Updates the readme with details on the OpenTelemetry SDKs builder
pattern and how it can lead to options set by the distro to be
overridden.

- Closes #350 

## Short description of the changes
Update README with OTel SDK builder pattern

README.md

@@ -26,6 +26,13 @@ Honeycomb's OpenTelemetry .NET SDK gives you the ability to add manual instrumen
 - Deterministic sampling!
 - Multi-span attributes!
 
+### Overriding OpenTelemetry SDK Builder options
+
+The OpenTelemetry SDK uses a builder pattern to set options and currently does not provide a way to know if a particular option has already been set.
+This can lead to the same option being set multiple times with the last one wins behaviour.
+
+For example, the `AddHoneycomb(options)` function configures a Sampler so another call to `SetSampler(sampler)` will override the first sampler.
+
 ## License
 
 [Apache 2.0 License](./LICENSE).