Add missing Denite configuration to readme

[ryanoasis]

Required Info

• I have searched the issues for my issue and found nothing related and/or helpful

• I have read or at least glanced at the FAQ

• I have read or at least glanced at the Wiki

• What version of vim are you using? N/A

  • hint: vim --version

• What version of vim-devicons are you using? 0.9.1

  • hint: :echo webdevicons#version()

• Are you using vim from the terminal or a GUI vim? N/A

• Are you using Mac, Linux or Windows? N/A

Implementation:

Add a Denite section just like we have for unite:

" adding the custom source to denite 
let g:webdevicons_enable_denite = 1

see for example: https://github.com/ryanoasis/vim-devicons/blame/master/readme.md#L308-L311

[mohamadLy]

I can take it, if it's ok with you.

[ryanoasis]

@mohamadLy Thanks! sure it's all yours 😄

[mohamadLy]

@ryanoasis thanks for giving the chance. I've opened a PR for it. Any feedback is welcome as this is my first time.

[ryanoasis]

Closed/fixed via PR #217. Thanks again 👍