Mention low-memory devices on Next Steps (bug 1053235)

https://bugzilla.mozilla.org/show_bug.cgi?id=1053235

@robhudson @eviljeff quick r?

![screenshot 2014-08-29 10 53 59](https://cloud.githubusercontent.com/assets/211578/4092741/61f364d2-2f95-11e4-966a-673beb3c64e2.png)