Demonstrate parse_quote in the crate-level documentation

This macro is commonly useful for building a syntax tree and should be featured more prominently.

```rust
// Add a trait bound to every type parameter
for param in &mut generics.params {
if let GenericParam::Type(ref mut type_param) = *param {
type_param.bounds.push(parse_quote!(diesel::query_builder::QueryId));
}
}
```