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:

• https://github.com/rustwasm/wasm-pack/issues/886
• https://github.com/rustwasm/wasm-pack/issues/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