Improve readme

README.md

@@ -4,7 +4,21 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/raviqqe/linkcheck?style=flat-square)](https://goreportcard.com/report/github.com/raviqqe/linkcheck)
 [![License](https://img.shields.io/github/license/raviqqe/linkcheck.svg?style=flat-square)](LICENSE)
 
-Check links in Markdown or HTML
+`linkcheck` is a command line tool to check links in Markdown and HTML files.
+It checks all `a` and `img` tags in specified files.
+
+## Installation
+
+```sh
+go get -u github.com/raviqqe/linkcheck
+```
+
+## Usage
+
+```sh
+linkcheck file.md
+linkcheck file1.md file2.md
+```
 
 ## License