Added description of what Topshelf is

Hi, I find it very useful when I'm reading an unfamiliar code base to have a line or two describing the reason for the existence of a library/framework.

I found the phrase I added to your readme in the 18th link down in the documentation hidden under "Overview". (BTW all the links in http://topshelf-project.com/ don't go anywhere and you have lorem ipsum in one of your quotes.)

That phrase describes your project beautifully and I now fully understand why my company is using it.

Please put that phrase up-front on EVERYTHING!

Thanks.

Kevin.