Make a note about fn render() problem and point a fix

[orsenkucher]

Summary

Today, there is a catch with pub fn render(&self) -> String method. String return type crashes wasm-pack build on wasm-pack 0.9.1 version, see:

• wasm-opt: Exported global cannot be mutable wasm-pack#886
• build failed when converting .rs to .wasm wasm-pack#900

I've made a note on it, with suggested fix. Hope this will help other readers to resolve it quickly, still mentioning the issue explicitly