v2: Extract package level documentation to doc.go (#303)

* Extract package level documentation to doc.go

Plus elaborate the documentation a bit.

* Fix grammar

doc.go

@@ -0,0 +1,18 @@
+// Package blackfriday is a markdown processor.
+//
+// It translates plain text with simple formatting rules into an AST, which can
+// then be further processed to HTML (provided by Blackfriday itself) or other
+// formats (provided by the community).
+//
+// The simplest way to invoke Blackfriday is to call one of Markdown*
+// functions. It will take a text input and produce a text output in HTML (or
+// other format).
+//
+// A slightly more sophisticated way to use Blackfriday is to call Parse, which
+// returns a syntax tree for the input document. You can use that to write your
+// own renderer or, for example, to leverage Blackfriday's parsing for content
+// extraction from markdown documents.
+//
+// If you're interested in calling Blackfriday from command line, see
+// https://github.com/russross/blackfriday-tool.
+package blackfriday

markdown.go

@@ -1,21 +1,10 @@
-//
 // Blackfriday Markdown Processor
 // Available at http://github.com/russross/blackfriday
 //
 // Copyright © 2011 Russ Ross <russ@russross.com>.
 // Distributed under the Simplified BSD License.
 // See README.md for details.
-//
 
-//
-//
-// Markdown parsing and processing
-//
-//
-
-// Package blackfriday is a markdown processor.
-//
-// Translates plain text with simple formatting rules into HTML or LaTeX.
 package blackfriday
 
 import (
@@ -26,6 +15,10 @@ import (
 	"unicode/utf8"
 )
 
+//
+// Markdown parsing and processing
+//
+
 // Version string of the package.
 const Version = "2.0"