Documentation: Explain we only support current version & push updates

Per https://github.com/JulietaUla/Montserrat/issues/60#issuecomment-342669948 and https://github.com/google/fonts/issues/1307 we should update our about page (and FAQ) to tell users that

1. we push family updates that improve the quality of the fonts but may change their visual design ~~in subtle ways~~ in ways you may not expect;
2. while an old version's files may be available if accessed directly from the gstatic CDN, they should not be directly linked to or relied upon, because they could disappear at any time - only the CSS API is supported; and
3. if people really care about controlling updates, they should self-host; likely with something like an NPM package, which gives finer version control and can track the actual font files' internal versioning scheme.