readme-Problem to solve

add item “Problem to solve”
revise "Problem"

make changes to make it’s possible to put two timeline plugins in one
page at the same time
Change name

1.change the name of the plugin from “timeline2” to “timeline”
2.change the js file’s name from “timeline2.js” to “timeline.js”
3.change the css file’s name from “theme.css” to “timeline-main.css”
change html name