docs: add example with auto detection of SDL2 version

[larpon]

README.md render of PR here: https://github.com/Larpon/sdl/tree/208/docs/add-auto-sdl2-version-example

[spytheman]

post_setup.vsh

import os

os.chdir(os.dir(os.executable()))!

res := os.execute('sdl2-config --version')
if res.exit_code != 0 { println('sdl2-config is missing') exit(1) }
system_version := res.output.trim_space()
println('Your version is $system_version')

remotes := os.execute('git branch -r --list')
if remotes.exit_code != 0 { println('git is missing') exit(1) }
supported_versions := remotes.output.split_into_lines().map(it.trim_space()).filter(it.starts_with('origin/2')).map(it.all_after('origin/'))
println('The SDL module officially supports these versions of SDL:\n   $supported_versions')

if system_version in supported_versions {
	println('Setting up the repository to branch $system_version that exactly matches your system SDL version')
	os.system('git checkout $system_version')
	exit(0)
}

base_version := '${system_version.all_before_last('.')}.0'
println('Setting up the repository to branch $base_version, that best matches the system SDL version: $system_version ...')
os.system('git checkout $base_version')

[spytheman]

I thought that if we already have portable V, then why not use it and make it a little bit more maintainable :-) .

Usage: v ~/.vmodules/sdl/post_setup.vsh .

[larpon]

Super nice addition 💯