Fix errors in README.md

[heyuanw-dev]

Hi, this is for my class assignment. I fixed some errors in the writing:

• add "Languages: " in front of different versions of the file to make it clear
• add a "Feature" section
• put all the functions into a list for better readability
• rewrite introduction
• "singleSelection, Multiselect" -> "Single and multiple selection of photos"
• "scalable" -> "Scaling photos"
• "sliding" -> "slide"
• "configuration" -> "Configuration" as it is a heading