Not clear that vim devicons needs to be loaded before vim-buffet

[Akin]

Firstly thanks for the great plugin 💯 it's been super useful.
When I initially installed it I tried to get it working with vim devicons but kept finding that the icons wouldn't show. I re-read the instructions in the README and realised (although it isn't explicitly stated) that I had to load vim devicons before vim buffet

Plug 'vim-devicons' "<-- once move here it works fine
Plug 'vim-buffet'
Plug 'vim-devicons' "<-- Initially below buffet, doesn't load icons

This is probably not too big a deal but I think it would be clearer if the README.md was explicit about this or maybe the plugin could do something similar to lightline-bufferline which seems to be able to load the devicons regardless of the positioning.

[Bagrat Aznauryan]

@Akin909 thanks for pointing this out! I have updated the README to address that.