Modifying the readme in example to address the wayland default

[vivekadishankara]

I am new to bevy, so I just cloned the repository and did a cargo build. That gave me the error:

/home/einfochips/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wayland-sys-0.31.7/build.rs:10:47:
  called `Result::unwrap()` on an `Err` value: Failed to run command `PKG_CONFIG_ALLOW_SYSTEM_LIBS=1 PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags wayland-client`, because: Not a directory (os error 20)

then I discovered that my system runs on x11, but when i ran the example:

cargo run --features x11 --example hello_world

I still got the error.
This PR is trying to fix this.

Objective

Since engine runs on wayland by default, the defaults need to be disabled for running on x11.

Solution

The first example of cargo run in example/README.md needs to specify how to do this. Hence a new example is added.

Testing

I did run the cargo my self and the newly added example works on my system, the command being:

cargo run --no-default-features --features x11 --example hello_world

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[alice-i-cecile]

You'll need to fix CI, but this seems helpful and the wording is good.

[vivekadishankara]

This might not be a good idea because when i tried to run other examples they failed to compile because other default features were required

[alice-i-cecile]

Related to #21158; I think this needs to be blocked on that.

[vivekadishankara]

There is one problem in the current condition. The only way i can run the examples is to manually comment out the wayland default in the cargo toml. Currently there is no way to disable only one default.

[vivekadishankara]

I jumped to a wayland system now