Composer installation instructions update

[sgort]

Issues found:

1. composer source command doesn't exist in Composer
2. Plugin requires both custom autoloader AND Composer autoloader, but only includes the custom one
3. Development dependencies require PHP tokenizer extension that may not be available on all systems

Solutions provided:

1. Alternative installation methods that actually work
2. Required autoloader fix to prevent fatal errors
3. Use of --no-dev flag to avoid unnecessary development dependencies

This should help other users avoid the same installation issues I encountered!