os: add a simple example to the package doc.
Shows error handling and slices for Read and Write.
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5676075
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 4391642..ddcaa6f 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -7,11 +7,33 @@
// Go-like; failing calls return values of type error rather than error numbers.
// Often, more information is available within the error. For example,
// if a call that takes a file name fails, such as Open or Stat, the error
-// will include failing file name when printed and will be of type *PathError,
-// which may be unpacked for more information.
+// will include the failing file name when printed and will be of type
+// *PathError, which may be unpacked for more information.
//
// The os interface is intended to be uniform across all operating systems.
// Features not generally available appear in the system-specific package syscall.
+//
+// Here is a simple example, opening a file and reading some of it.
+//
+// file, err := os.Open("file.go") // For read access.
+// if err != nil {
+// log.Fatal(err)
+// }
+//
+// If the open fails, the error string will be self-explanatory, like
+//
+// open file.go: no such file or directory
+//
+// The file's data can then be read into a slice of bytes. Read and
+// Write take their byte counts from the length of the artument slice.
+//
+// data := make([]byte, 100)
+// count, err := file.Read(data)
+// if err != nil {
+// log.Fatal(err)
+// }
+// fmt.Printf("read %d bytes: %q\n", count, data[:count])
+//
package os
import (
