message: Add graphics::present to basic project template
  It's required 99% of the time, and easy to forget as a newcomer. I also
  added a line for `graphics::clear`, because it's often needed too.
diff: //! ```rust,no_run
  //! use ggez::{Context, ContextBuilder, GameResult};
  //! use ggez::event::{self, EventHandler};
  +//! use ggez::graphics;
  //!
  //! fn main() {
  //! // Make a Context and an EventLoop.
  //! # Ok(())
  //! }
  //!
  -//! fn draw(&mut self, _ctx: &mut Context) -> GameResult<()> {
  +//! fn draw(&mut self, ctx: &mut Context) -> GameResult<()> {
  +//! // Optional
  +//! graphics::clear(ctx, graphics::WHITE);
  +//!
  //! // Draw code here...
  -//! # Ok(())
  +//!
  +//! graphics::present(ctx)
  //! }
  //! }
  //! #
  