Add README.md

Author: noah

README.md

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+# Cron expression parser
+
+**Given a cron expression, you can get the previous timestamp or the next timestamp that satisfies the cron expression.**
+
+I have used cron expression syntax to implement a new feature called *deploy freeze window* in Gitploy.
+
+## Install
+
+```shell
+go get github.com/gitploy-io/cronexpr
+```
+
+## Usage
+
+Import the package, first:
+
+```go
+import "time"
+import "github.com/gitploy-io/cronexpr"
+```
+
+```go
+prevTime := cronexpr.MustParse("0 0 29 * *").Prev(time.Now())
+nextTime := cronexpr.MustParse("0 0 29 * *").Next(time.Now())
+```
+
+You can check the detail in the `_example` directory.
+
+
+## Implementation
+
+
+```
+Field name     Mandatory?   Allowed values    Allowed special characters
+----------     ----------   --------------    --------------------------
+Minutes        Yes          0-59              * / , -
+Hours          Yes          0-23              * / , -
+Day of month   Yes          1-31              * / , - 
+Month          Yes          1-12 or JAN-DEC   * / , -
+Day of week    Yes          0-6 or SUN-SAT    * / , - 
+```
+
+### Asterisk ( * )
+The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month. 
+
+### Slash ( / )
+Slashes describe increments of ranges. For example `3-59/15` in the minute field indicate the third minute of the hour and every 15 minutes thereafter. The form `*/...` is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field.
+
+### Comma ( , )
+Commas are used to separate items of a list. For example, using `MON,WED,FRI` in the 5th field (day of week) means Mondays, Wednesdays and Fridays.
+
+### Hyphen ( - )
+Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.
+

test/main.go renamed to _example/freeze.go

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+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/gitploy-io/cronexpr"
+)
+
+func main() {
+	t := time.Date(2013, 8, 29, 9, 28, 0, 0, time.UTC)
+
+	nextTime := cronexpr.MustParse("0 0 29 2 *").Next(t)
+	fmt.Print(nextTime)
+}