adding installing and usage sections in README

Author: ellisonleao

README.md

@@ -1,3 +1,22 @@
 # verify-links
 
 CLI tool that helps verify current status of URIs in text files
+
+# Installing
+
+```
+go get github.com/npxbr/verify-links/cmd/vl
+```
+
+# Usage
+
+```
+  -a string (Skip status code list)
+        -a 500,400
+  -s int (Concurrency size)
+        -s 50 (default 50)
+  -t duration (max timeout for each request)
+        -t 10s or -t 1h (default 5s)
+  -w string (whitelist URI list)
+        -w server1.com,server2.com
+```