add readme and a screenshot

Author: zserge

README.md

@@ -4,3 +4,54 @@
 [![GoDoc](https://godoc.org/github.com/zserge/metric?status.svg)](https://godoc.org/github.com/zserge/metric)
 [![Go Report Card](https://goreportcard.com/badge/github.com/zserge/metric)](https://goreportcard.com/report/github.com/zserge/metric)
 
+Package provides simple uniform interface for metrics such as counters,
+gauges and histograms. It keeps track of metrics in runtime and can be used for
+some basic web service instrumentation in Go, where complex tools such as
+Prometheus or InfluxDB are not required.
+
+It is compatible with [expvar](https://golang.org/pkg/expvar/) package, that is
+also commonly used for monitoring.
+
+## Usage
+
+```go
+// Create new metric. All metrics may take time frames if you want them to keep
+// history. If no time frames are given the metric only keeps track of a single
+// current value.
+c := metric.NewCounter("15m10s") // 15 minutes of history with 10 second precision
+// Increment counter
+c.Add(1)
+// Return JSON with all recorded counter values
+c.String() // Or json.Marshal(c)
+
+// With expvar
+
+// Register a metric
+expvar.Publish("latency", metric.NewHistogram("5m1s", "15m30s", "1h1m"))
+// Register HTTP handler to visualize metrics
+http.Handle("/debug/metrics", metric.Handler(metric.Exposed))
+
+// Measure time and update the metric
+start := time.Now()
+...
+expvar.Get("latency").(metric.Metric).Add(time.Since(start).Seconds())
+```
+
+Metrics are thread-safe and can be updated from background goroutines.
+
+## Web UI
+
+Nothing fancy, really, but still better than reading plain JSON. No javascript,
+only good old HTML, CSS and SVG.
+
+![web ui](example/screenshot.png)
+
+Of course you may customize a list of metrics to show in the web UI.
+
+If you need precise values - you may use `/debug/vars` HTTP endpoint provided
+by `expvar`.
+
+## License
+
+Code is distributed under MIT license, feel free to use it in your proprietary
+projects as well.