Improve documentation on configuring plugin

[MariaMozgunova]

I recently went through the task of installing and configuring the OpenID Connect Server plugin on the WordPress website. I faced a few issues along the way. Reviewing the 'Genning started' issue greatly helped me. I want to contribute to improving the README.md file and make the installation process easier for future users.

• I elaborated on where to add define statements
• What is the easiest way to add a filter for defining clients
• How to check plugin activation is successful