chore(docs): Improved CLI Examples (#2303)

Author: alexandercerutti

README.md

@@ -47,14 +47,21 @@ Also read about:
 
 **CLI**
 
+
 ``` bash
+# Example with stdin input
 $ marked -o hello.html
 hello world
 ^D
 $ cat hello.html
 <p>hello world</p>
 ```
 
+```bash
+# Print all options
+$ marked --help
+```
+
 **Browser**
 
 ```html

docs/INDEX.md

@@ -30,6 +30,7 @@ These documentation pages are also rendered using marked 💯
 **CLI**
 
 ``` bash
+# Example with stdin input
 $ marked -o hello.html
 hello world
 ^D
@@ -38,10 +39,26 @@ $ cat hello.html
 ```
 
 ``` bash
+# Example with string input
 $ marked -s "*hello world*"
 <p><em>hello world</em></p>
 ```
 
+```bash
+# Example with file input
+
+echo "**bold text example**" > readme.md
+
+$ marked -i readme.md -o readme.html
+$ cat readme.html
+<p><strong>bold text example</strong></p>
+```
+
+```bash
+# Print all options
+$ marked --help
+```
+
 **Browser**
 
 ```html