Add System Metrics docs to README (#5)

Author: MrLotU

README.md

@@ -40,4 +40,54 @@ Swift Metrics Extras ships the following extra modules:
 
 ### System Metrics
 
-TODO: some docs about the system metrics.
+The System Metrics package provides default process metrics for applications. The following metrics are exposed:
+
+- Virtual memory in Bytes.
+- Resident memory in Bytes.
+- Application start time in Seconds.
+- Total CPU seconds.
+- Maximum number of file descriptors.
+- Number of file descriptors currently in use.
+
+***NOTE:*** Currently these metrics are only implemented on Linux platforms, and not on Darwin or Windows.
+
+#### Using System Metrics
+
+After [adding swift-metrics-extras as a dependency](#adding-the-dependency) you can import the `SystemMetrics` module.
+
+```swift
+import SystemMetrics
+```
+
+This makes the System Metrics API available. This adds a new method to `MetricsSystem` called `bootstrapWithSystemMetrics`. Calling this method will call `MetricsSystem.bootstrap` as well as bootstrapping System Metrics.
+
+`bootstrapWithSystemMetrics` takes a `SystemMetrics.Configuration` object to configure the system metrics. The config has the following properties:
+
+- interval: The interval at which SystemMetrics are being calculated & exported.
+- dataProvider: A closure returing `SystemMetrics.Data?`. When `nil` no metrics are exported (the default on non-linux platforms). `SystemMetrics.Data` holds all the values mentioned above.
+- labels: `SystemMetrics.Labels` hold a string label for each of the above mentioned metrics that will be used for the metric labels, along with a prefix that will be used for all above mentioned metrics.
+
+Swift Metrics backend implementations are encouraged to provide static extensions to `SystemMetrics.Configuration` that fit the requirements of their specific backends. For example:
+```swift
+public extension SystemMetrics.Configuration {
+    /// `SystemMetrics.Configuration` with Prometheus style labels.
+    ///
+    /// For more information see `SystemMetrics.Configuration`
+    static let prometheus = SystemMetrics.Configuration(
+        labels: .init(
+            prefix: "process_",
+            virtualMemoryBytes: "virtual_memory_bytes",
+            residentMemoryBytes: "resident_memory_bytes",
+            startTimeSeconds: "start_time_seconds",
+            cpuSecondsTotal: "cpu_seconds_total",
+            maxFds: "max_fds",
+            openFds: "open_fds"
+        )
+    )
+}
+```
+This allows end users to add System Metrics like this:
+
+```swift
+MetricsSystem.bootstrapWithSystemMetrics(myPrometheusInstance, config: .prometheus)
+```