Add documentation for CORS preflight request

[lprimak]

Additional documentation needed for apache/shiro#2372, to include

• how configure allow / disallow preflight requests
• how it works

[lprimak]

@celikfatih May I ask you to add documentation and some examples on how to turn on and use CORS preflight requests?
Thank you

[celikfatih]

@lprimak Yes, of course I can add it. I'll try to do that as soon as possible. Can you assign me?

[lprimak]

Hi, @celikfatih Happy new year!
Do you have some time soon to get this docs going?
Thank you!

[celikfatih]

Hi @lprimak, Happy New Year! Yes, I've added the changes. I'd like to ask you to check them.