add warning content about saving data.

[0x11-dev]

update readme.md. add warning content about saving data.

[0x11-dev]

hi there, I found the solution after hours debug and read session source code. I thought It may saving others time by add the warning content. Many users ask questions on StackOverflow.com when they face the same situation. This may be worth.

[dougwilson]

Thanks, but it's pretty much a duplicate of the existing docs:

This method is automatically called at the end of the HTTP response if the session data has been altered (though this behavior can be altered with various options in the middleware constructor).

[0x11-dev]

The existing docs does not address the problem out: Update session after response.send or response.end will not work. BTW, the existing docs is very elegant. Professionals will notice when they saw it, but the beginner may not.