Add libcurl installation advice to documentation

[epogrebnyak]

It is good to have an explicit advice to install curl libraries in the docsting of the package somewhere around here.

The docstring in Network.Curl mentions package is a binding, but does not make any explicit suggestion on how to install it.

Text suggestion for Network.Curl documentation:

On linux you can install libcurl libraries with sudo apt-get install libcurl4-gnutls-dev or similar command appropriate for your system.

Note this question on Stackoverflow directs to sudo apt-get install libcurl4-gnutls-dev and got 15 upvotes which is quite a bit for this type of question - that means people are faced with a problem and look for an answer. Someone also says they'd be happy to see this advice in the documentation.

In my personal experience someone trying to get things done in Haskell if faced with multiple problems of understanding the language, the ecosystem, the tooling, so taking one question about the bidnings off the user worries list would be very appreciated.

I can submit a PR to Network/Curl.hs if the wording above seems appropriate.

Would also close #20.