Change project description to make it clear external session stores can be used

[natesilva]

We were using a different session module. I never looked at koa-session because the description sounds like it only supports cookies (we use Redis). Turns out, it’s easy to plug Redis into koa-session.

It would be helpful to edit the project description on Github and package.json so dummies like me will give this a try. This is the most-downloaded Koa session middleware and should be the go-to module for all session needs.

[dead-horse]

npm description changed. :)

thanks!