Add documentation

Author: timonwong

README.md

@@ -1,5 +1,10 @@
 # go-strftime
 
+[![GoDoc](https://godoc.org/github.com/imperfectgo/go-strftime?status.svg)](https://godoc.org/github.com/imperfectgo/go-strftime) 
+[![Build Status](https://travis-ci.org/imperfectgo/go-strftime.svg?branch=master)](https://travis-ci.org/imperfectgo/go-strftime)
+[![Go Report Card](https://goreportcard.com/badge/github.com/imperfectgo/go-strftime)](https://goreportcard.com/report/github.com/imperfectgo/go-strftime)
+[![Coverage](https://codecov.io/gh/imperfectgo/go-strftime/branch/master/graph/badge.svg)](https://codecov.io/gh/imperfectgo/go-strftime)
+
 High performance C99-compatible `strftime` formatter for Go.
 
 **EXPERIMENTAL** Please DO NOT USE for now.

format.go

@@ -49,6 +49,8 @@ const (
 	stdMask                = 1<<stdArgShift - 1           // mask out argument
 )
 
+// Format returns a textual representation of the time value formatted
+// according to C99-compatible strftime layout.
 func Format(t time.Time, layout string) string {
 	const bufSize = 64
 	var b [bufSize]byte
@@ -57,6 +59,8 @@ func Format(t time.Time, layout string) string {
 	return string(buf)
 }
 
+// AppendFormat is like Format but appends the textual
+// representation to b and returns the extended buffer.
 func AppendFormat(b []byte, t time.Time, layout string) []byte {
 	var (
 		name, offset, abs = locabs(&t)