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
Use Python syntax highlighting for .wsgi file extensions, ie. Python web apps using mod_wsgi.
Using variables for warning colors.
Merge branch 'master' of https://github.com/adobe/brackets into larz/issue-4392
Remove default extension from untitled documents and new files
Merge pull request #4483 from adobe/iwehrman/issue-4481

Remove default extension from untitled documents and new files.  Avoids Tern issues with untitled documents (e.g. #4481).
Merge pull request #4375 from tomByrer/patch-1

reorganize Readme
Update README.md
Merge pull request #4482 from adobe/larz/issue-4392

Larz/issue 4392
Add syntax wirings for .scala, .sbt, .properties, and .ini
Merge pull request #4431 from globexdesigns/master

[OPEN].wsgi files should use Python syntax highlighting