Spruce up VisualEditor's README file.

Convert to Markdown, for better rendering in github.  Link more
prominently to install instructions in the README, also mention
Gerrit and the code style and API documentation.

Change-Id: I8e26fea108c32f37078e363f1af18eaf28b83516

README.md

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+# VisualEditor
+
+VisualEditor provides a visual editor for wiki pages. It is written in
+JavaScript and runs in a web browser.
+
+It uses the Parsoid parser to convert wikitext documents to annotated HTML
+which the VisualEditor is able to load, modify and emit back to Parsoid at
+which point it is converted back into wikitext.
+
+For more information about these projects, check out the [VisualEditor][]
+and [Parsoid][] pages on mediawiki.
+
+
+## Developing and installing
+
+For information on installing VisualEditor on a local wiki, please
+see https://www.mediawiki.org/wiki/Extension:VisualEditor
+
+For information about contributing code to VisualEditor, see
+[CODING.md][].  Patch submissions are reviewed and managed with
+[Gerrit][].  There is also [API documentation][] available for the
+VisualEditor.
+
+[VisualEditor]:      http://www.mediawiki.org/wiki/VisualEditor
+[Parsoid]:           http://www.mediawiki.org/wiki/Parsoid
+[CODING.md]:         CODING.md
+[API documentation]: https://doc.wikimedia.org/VisualEditor/master/
+[Gerrit]:            https://www.mediawiki.org/wiki/Gerrit

README.txt

@@ -1,11 +0,0 @@
-VisualEditor provides a visual editor for wiki pages. It is written in
-JavaScript and runs in a web browser.
-
-It uses the Parsoid parser to convert wikitext documents to annotated HTML
-which the VisualEditor is able to load, modify and emit back to Parsoid at
-which point it is converted back into wikitext.
-
-For more information about these projects, check out the wiki:
-
-	* http://www.mediawiki.org/wiki/VisualEditor
-	* http://www.mediawiki.org/wiki/Parsoid