add trait example

Author: yggverse

README.md

@@ -32,6 +32,8 @@ for line in gemtext.lines() {
 
 ##### Inline
 
+**Struct**
+
 ``` rust
 use ggemtext::line::code::Inline;
 match Inline::from("```inline```") {
@@ -40,6 +42,14 @@ match Inline::from("```inline```") {
 }
 ```
 
+**Trait**
+
+``` rust
+use ggemtext::line::code::inline::Gemtext;
+assert_eq!("```inline```".as_value(), Some("inline"))
+assert_eq!("inline".to_source(), "```inline```")
+```
+
 ##### Multiline
 
 ``` rust