Update documentation with available message customizations

[gutomotta]

Hi,

Thank's for the good job on the lib.

I'd like to suggest an update in the library documentation, specifically at this page:
https://github.com/Mobius1/Selectr/wiki/Options#messages

There are two customizable messages that are not yet documented: noOptions and searchPlaceholder.

I'd update the Wiki myself but unfortunately AFAIK Github does not allow pull-requests on Wiki's.

[PawelGlow]

+1