Add content from Sites to Atom guide.

This CL adds the relevant content at
https://www.chromium.org/developers/using-atom-as-your-ide to the Atom
guide at docs/atom.md, so the Sites page can be replaced with a link to
the Markdown guide.

BUG=

Review-Url: https://codereview.chromium.org/2557613002
Cr-Commit-Position: refs/heads/master@{#436497}

docs/atom.md

@@ -1,5 +1,16 @@
 # Atom
 
+[Atom](https://atom.io/)
+([Wikipedia](https://en.wikipedia.org/wiki/Atom_(text_editor))) is a
+multi-platform code editor that is itself based on Chromium.
+[Turtles aside](https://en.wikipedia.org/wiki/Turtles_all_the_way_down), Atom
+has a growing community and base of installable plugins and themes.
+
+You can download and install via links from the
+[main Atom site](https://atom.io/). If you're interested in checking out the
+code and contributing, see the
+[developer page](https://github.com/atom/atom/blob/master/docs/build-instructions/linux.md).
+
 [TOC]
 
 ## Workflow
@@ -21,7 +32,7 @@ A typical Atom workflow consists of the following.
 To setup this workflow, install Atom packages for Chrome development.
 
 ```
-$ apm install build-ninja clang-format \
+$ apm install build build-ninja clang-format \
     linter linter-cpplint linter-eslint switch-header-source you-complete-me
 ```