Update README.md to clarify installation and usage instructions

README.md

@@ -1,5 +1,21 @@
 # letsgo - Local Enumeration & Traversal Scanning in Go
 
-# Installation
+# Installation && Update
+
+Install and update use the same command, just run the following command then everything's done.
+
+```bash
+go install github.com/CX330Blake/letsgo@latest
+```
 
 # Usage
+
+Use `letsgo --help` to see the detail usage.
+
+## Basic usage
+
+Use the default settings to scan a URL.
+
+```bash
+letsgo --url <https://example.com>
+```