Instructions for building and developing?

[justindbelanger]

Hi, @susielu, I've started using react-annotation in a project and I'm really enjoying the broad range of features that it offers! However, I've been trying to build the library and experiment with some of my own changes, and I'm having some trouble. When I first tried to build the project, I didn't realize that I had to use yarn instead of npm. Also, I'm wondering, what's the best way to test out changes I've made to this library? I think both of these issues could be addressed in a new section of the 'README.md'. What do you think?