Add README to example-showcase tool

[alice-i-cecile]

Confusion and answers start here: https://discord.com/channels/691052431525675048/743663924229963868/1391561061349593173

What:

it's the tool that power https://bevyengine.github.io/bevy-example-runner/ among other things
and https://bevy.org/examples/

Why:

because CI nodes are not very powerful and the default config is too much for them

Why patching:

why do we rely on patching for this?
because it can't modify the example code