reorganize Readme

Moved paragraphs around so least technical (most user-oriented) is on top, then tech/contributing (hacker info) is near the end.  This allowed to streamline the prose, but I tried to avoid changing the text as much as possible.

Linked blog in welcome section (great for finding features; helps users decide if Brackets is for them).

Used H4 vs **bold** for sub-headers.
more terse, less repeated text

(note to self; don't Push with only 4 hours of sleep)
more terse, less repeated text, ordered helping steps
reorganize getting help
Merge pull request #4375 from tomByrer/patch-1

reorganize Readme