js/README.md grammar errors

Created by: cvrebert

All plugins should have a default object which can be modified to _effect_ all _instance's_ default options.

Should be:

All plugins should have a default object which can be modified to _affect_ all _instances'_ default options.

(or "instances's", depending on personal preference)

Also:

Each plugin should expose _it's_ raw constructor

Should be:

Each plugin should expose _its_ raw constructor