Update README with babel plugin ordering

[mgcrea]

Had a hard time today figuring out why a very simple mobx react sandbox app did not work.

Turned out that the issue was due to the order of babel plugins in my .babelrc.

transform-decorators-legacy has to appear first in the list, otherwise it will break with at least either transform-function-bind or transform-class-properties plugins if they defined before it, with the wrong orders, observables are not triggered at all.

Would be great to update README to clarify this.

[mweststrate]

wow, never noticed that the order is relevant. thanks for reporting, I'll update it

[mgcrea]

Maybe that's something that mobx could detect/catch? (like if the class property decorator gets a function instead of an object/array ref), would be great to add a console warning if it can be safely detected.

[mweststrate]

good one, I'm gonna check whether it is detectable

[samhh]

Thank you so much for posting this, I've been tearing my hair out trying to understand why nothing was working.