Adds mapping for select_next_item and select_next_item in README

[cskeeters]

This adds mappings for and in the proposed configuration. Without these mappings changes the pop-up menu to some other set of completions (built-in buffer words?) and the plugin does not work properly.

Detailed example: If i have snippets named logd and loge and I type le, both options will show in the menu due to fuzzy matching, but when I press to select the next item in the list, the list of options changes and I can't select the loge snippet.

[cskeeters]

Nevermind, my problem was that I didn't use cmp.mapping.preset.insert