Configuration: more advanced Vim / Coc example (suggestion)

[andys8]

I wasn't aware how to properly configure CoC language servers so that they're recognized by haskell language server (see #3157 (comment)). Might be possible that the configuration changed. Therefore I'd suggest to add an example showing how to configure settings for plugins. I chose to add a second example in order to keep the default example minimal and avoid confusing people who want an easy way to start with.

This suggestion is open for discussion. I can see how this could somehow be redundant, or more than necessary for a minimal example. At the same time when I did want to look up how the configuration should be defined, I didn't find any information.

By the way, I also updated the example in the CoC wiki

[michaelpj]

I think this is good! That section is light because a) we need someone who actually knows CoC to fill it in (which it seems like you do, which is great) and b) we want to avoid duplicating information from the client documentation. But I think having a slightly bigger example here is great!