docs: update readme with usage and description

Signed-off-by: m-murad <murad.kuka@gmail.com>

readme.md

@@ -1,2 +1,44 @@
-## once
+## sync-once
 
+sync-once is similar to [sync.Once](https://golang.org/pkg/sync/#Once) of the standard library.
+
+It also has struct `Once` but has two additional methods `DoAgain()` and `Reset()`.
+
+### why
+
+While writing a web application I needed to reload configurations that were calculated using sync.Once. 
+
+But `sync.Once` provide a single method `Do()` that executes the function only once.
+
+To get around this I wrote this package.
+
+### usage
+
+```
+import (
+    sync "github.com/m-murad/sync-once"
+)
+
+func main() {
+    o := new(sync.Once)
+    
+    // This will work similar to the Once.Do(f) method of the sync package. The function f is only called once
+    o.Do(loadConfig())
+
+    // To call the function either for the first time or again you need to use the sync.DoAgain() method
+    // This will execute irrespective of weather o.Do() was called earlier or not and mark o (Once) as done.
+    // Call to o.Do() after o.DoAgain() will not execute the function.
+    o.DoAgain(loadConfig())
+
+    // To reset o (sync.Once) you need to call the Reset() method.
+    // This will mark o as not executed but will no call the Do() method. You need to call Do() or DoAgain() after this.
+    // Calls to Do() and DoAgain() after this will work as described above.
+    o.Reset()
+
+}
+
+// load config from a static file or any other operation that is usually performed only once
+func loadConfig() {
+    // Do the work here
+}
+```