Update README
Update README

Fix/improve spelling, grammar, markdown format, and links.

The css file was removed in <https://github.com/RenovoSolutions/ngx-datetimepicker/pull/70>, so added a note about compiling it first.

RenovoSolutions/ngx-datetimepicker,RenovoSolutions/ngx-datetimepicker,RenovoSolutions/ngx-datetimepicker