Improve the README (#3)

* Improve the README

* Slight reword

README.md

@@ -5,5 +5,69 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/oklog/run)](https://goreportcard.com/report/github.com/oklog/run)
 [![Apache 2 licensed](https://img.shields.io/badge/license-Apache2-blue.svg)](https://raw.githubusercontent.com/oklog/run/master/LICENSE)
 
-A universal mechanism to manage goroutine lifecycles.
+run.Group is a universal mechanism to manage goroutine lifecycles.
 
+Create a zero-value run.Group, and then add actors to it. Actors are defined as
+a pair of functions: an **execute** function, which should run synchronously;
+and an **interrupt** function, which, when invoked, should cause the execute
+function to return. Finally, invoke Run, which blocks until the first actor
+returns. This general-purpose API allows callers to model pretty much any
+runnable task, and achieve well-defined lifecycle semantics for the group.
+
+run.Group was written to manage component lifecycles in func main for 
+[OK Log](https://github.com/oklog/oklog). 
+But it's useful in any circumstance where you need to orchestrate multiple
+goroutines as a unit whole.
+[Click here](https://www.youtube.com/watch?v=LHe1Cb_Ud_M&t=15m45s) to see a
+video of a talk where run.Group is described.
+
+## Examples
+
+### context.Context
+
+```go
+ctx, cancel := context.WithCancel(context.Background())
+g.Add(func() error {
+	return myProcess(ctx, ...)
+}, func(error) {
+	cancel()
+})
+```
+
+### net.Listener
+
+```go
+ln, _ := net.Listen("tcp", ":8080")
+g.Add(func() error {
+	return http.Serve(ln, nil)
+}, func(error) {
+	ln.Close()
+})
+```
+
+### io.ReadCloser
+
+```go
+var conn io.ReadCloser = ...
+g.Add(func() error {
+	s := bufio.NewScanner(conn)
+	for s.Scan() {
+		println(s.Text())
+	}
+	return s.Err()
+}, func(error) {
+	conn.Close()
+})
+```
+
+## Comparisons
+
+Package run is somewhat similar to package 
+[errgroup](https://godoc.org/golang.org/x/sync/errgroup), 
+except it doesn't require actor goroutines to understand context semantics.
+
+It's somewhat similar to package
+[tomb.v1](https://godoc.org/gopkg.in/tomb.v1) or 
+[tomb.v2](https://godoc.org/gopkg.in/tomb.v2),
+except it has a much smaller API surface, delegating e.g. staged shutdown of 
+goroutines to the caller.