Haskell: Hello World client and server

[sohang3112]

Implemented Hello World client and server in Haskell

PS: The other language examples had auto-generated download.md files. I didn't include download.md in Haskell example directory as I wasn't sure where to make changes so that it would be auto-generated.

[sohang3112]

The checks above are failing because the Ubuntu version used in the checks is no longer supported. This is the error message in Netlify logs:

UNSUPPORTED BUILD IMAGE

  The build image for this site uses Ubuntu 16.04 Xenial Xerus, which is no longer supported.

  To enable builds for this site, select a later build image at the following link:
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[sohang3112]

@sappo these Haskell example codes are not yet showing at zeromq.org. Please re-build / re-host the site so that the site is updated.

[sohang3112]

@sappo Please re-host the site so that these added Haskell example codes show at zeromq.org