Readme updates

Author: MrLotU

README.md

@@ -27,6 +27,22 @@ let myProm = PrometheusClient()
 ## Usage with SwiftMetrics
 _For more details about swift-metrics, please view [the GitHub repo](https://github.com/apple/swift-metrics)._
 
+Starting with SwiftPrometheus [1.0.0-alpha.10](https://github.com/MrLotU/SwiftPrometheus/releases/tag/1.0.0-alpha.10) `MetricsSystem` is no longer directly configured with a `PrometheusClient`.
+
+Instead, create a `PrometheusMetricsFactory` instance wrapping a `PrometheusClient`.
+
+```swift
+let myProm = PrometheusClient()
+MetricsSystem.bootstrap(PrometheusMetricsFactory(client: myProm))
+```
+
+Along with a `PrometheusClient`, `PrometheusMetricsFactory` can take a `Configuration` object setting the following properties:
+- A `LabelSanitizer` used to sanitize metric names to valid Prometheus values. A default implementation is provided.
+- The Prometheus metric type to use for swift-metrics' `Timer`. Can be a `Histogram` or a `Summary`. Note that when using `Histogram`, `preferredDisplayUnit` will not be observed.
+- Default buckets for use by aggregating swift-metrics `Recorder` instances.
+
+### Before Alpha 10
+
 To use SwiftPrometheus with swift-metrics, you need to configure the backend inside the `MetricsSystem`:
 
 ```swift