Add haddock syntax guildline in Drasil Wiki

[cd155]

Drasil source code uses Haddock to generate code documentation. Haddock has a specific format to change Haskell's comments into a Haddock document. For example, the syntax below

-- |

Website https://haskell-haddock.readthedocs.io/en/latest/markup.html, gives the example and instructions. I think this information better included in the Drasil Wiki.

[smiths]

I agree @cd155 that this information would be useful. We could add it to the Contributor's Guide.

[cd155]

I added this part in Contributor's Guide - Coding Style