Clarify downstream browser support on gh-pages site

[tonypconway]

As discussed in this week's WebDX CG meeting, this draft adds a section in the main Baseline definition page that describes the relationship between Baseline and browsers outside the core set.

CC: @ddbeck @ai @rviscomi

[ddbeck]

I'm feeling positively toward this. I've got some ideas to make this a little tighter (and easier to maintain) in line suggestions. Thank you!

[ddbeck]

A few little suggestions for you, but I like how this is coming together!

[tonypconway]

@ddbeck I've folded in the wording/formatting changes you suggested. I'm a fan of the Oxford comma, but some people are extremely anti - I'll try to remember you're preference for next time ;-)

Ready for review when you are.

[ddbeck]

I'm happy with this. I'd like to get a little wider review on the very first paragraph before merging—I'll mention this in the WebDX chat—but I think this is very likely to merge in the next day or two. Thank you!

[tonypconway]

OK, my last little clean up changes are done, @ddbeck ready to merge when you are I believe.