Include containerWidth and containerHeight in readme

[louisjuliendo]

Hi @aullman, first of all I want to say thanks for this great library!

Do you think we could include containerWidth and containerHeight as part of the options in readme file?

I'm using this lib together with opentok-react and I was wondering why my layout data is not updated when I call the getLayout() on a dynamic-sized container (I passed in default options) until I found out that there are other options: container width & height (i.e undocumented) with default value of 640px for width and 480px for height.

[aullman]

Ah yes, will do. Thanks!