Add readme.md file

A new readme.md file is added to provide detailed information about usage of Keep Running application. The readme describes the purpose of the application, how to use it, its default settings, arguments and options. It also provides a use case example for better understanding.

readme.md

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+Keep Running
+---
+
+Keep running is a quick and easy way to **keep** a process **running** during development or other **non-production** tasks. 
+
+> Please for the love of sanity, do not use this is production...
+
+# Usage
+
+Below you will see the output of the `--help` command. 
+Do note that if you don't specify any `--per-*` flag, it defaults to `--per-minute=4`.
+
+```
+Usage: kr [OPTIONS] <COMMAND>
+
+Arguments:
+  <COMMAND>  
+
+Options:
+      --per-minute <PER_MINUTE>  [default: 0]
+      --per-hour <PER_HOUR>      [default: 0]
+      --delay <DELAY>            [default: 0]
+  -h, --help                     Print help
+  -V, --version                  Print version
+```
+
+# Use cases
+
+## Game servers
+
+/keep-running/ is a great tool to run, for example, a Modded minecraft server. 
+As it automatically restarts the server if a crash occurs.
+
+![img.png](assets/vault-hunters-3rd.png)
+