Add usage examples to README.

Author: Invis1bleReborn

README.md

@@ -1,11 +1,38 @@
-What Is My IP
-=============
+# What Is My IP
 
 Simple WhatIsMyIpAddress CLI Client.
 
-Usage
------
+## Usage
+
+```
+whatismyip 1.3.0
+Usage: whatismyip [options]
+
+-f, --file <value>      Output file (will be created if not exists)
+-i, --interval <value>  update interval (in Scala's Duration format)
+--help                  prints this usage text
+--version
+```
+
+### Examples
+
+#### Check and print every 1 minute (default behaviour)
 
 ```sh
 sbt run
 ```
+
+#### Write to the file instead of `stdout`
+
+```sh
+sbt "run --file=/tmp/whatismyip"
+```
+
+#### Use the custom interval
+
+```sh
+sbt "run --interval=2.minutes --file=/tmp/whatismyip"
+```
+
+See [scala.concurrent.duration.DurationConversions](https://www.scala-lang.org/api/current/scala/concurrent/duration/DurationConversions.html)
+for possible interval durations.