Add Rhai book.

doc/src/rust/strings.md

@@ -0,0 +1,21 @@
+`String` Parameters in Rust Functions
+====================================
+
+{{#include ../links.md}}
+
+Rust functions accepting parameters of `String` should use `&str` instead because it maps directly to [`ImmutableString`]
+which is the type that Rhai uses to represent [strings] internally.
+
+```rust
+fn get_len1(s: String) -> i64 { s.len() as i64 }            // <- Rhai will not find this function
+fn get_len2(s: &str) -> i64 { s.len() as i64 }              // <- Rhai finds this function fine
+fn get_len3(s: ImmutableString) -> i64 { s.len() as i64 }   // <- the above is equivalent to this
+
+engine.register_fn("len1", get_len1);
+engine.register_fn("len2", get_len2);
+engine.register_fn("len3", get_len3);
+
+let len = engine.eval::<i64>("x.len1()")?;                  // error: function 'len1 (&str | ImmutableString)' not found
+let len = engine.eval::<i64>("x.len2()")?;                  // works fine
+let len = engine.eval::<i64>("x.len3()")?;                  // works fine
+```