Document configuration of external editor links

Summary: Provide some documentation for this feature since it's not super
obvious how it works.

Test Plan: Generated documentation, read documentation.

Reviewers: btrahan, vrana, jungejason, nh

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1521

src/docs/userguide/external_editor.diviner

@@ -0,0 +1,46 @@
+@title User Guide: Configuring an External Editor
+@group userguide
+
+Setting up an external editor to integrate with Diffusion and Differential.
+
+= Overview =
+
+You can configure a URI handler to allow you to open files from Differential
+and Diffusion in your preferred text editor.
+
+= Configuring Editors =
+
+To configure an external editor, go to Settings -> Application Settings ->
+Display Preferences and set "Editor Link" to a URI pattern (see below). This
+will enable an "Open in Editor" link in Differential, and an "Edit" button in
+Diffusion.
+
+In general, you'll set this field to something like:
+
+  lang=uri
+  editor://open/?file=%f
+
+== Configuring: TextMate on OS X ==
+
+TextMate installs a ##txmt://## handler by default, so it's easy to configure
+this feature if you use TextMate.
+
+First, create a local directory with symlinks for each repository callsign. For
+example, if you're developing Phabricator, it might look like this:
+
+  /Users/alincoln/editor_links/ $ ls -l
+  ... ARC -> /Users/alincoln/workspace/arcanist/
+  ... P -> /Users/alincoln/workspace/phabricator/
+  ... PHU -> /Users/alincoln/workspace/libphutil/
+
+Then set your "Editor Link" to:
+
+  lang=uri
+  txmt://open/?url=file:///Users/alincoln/editor_links/%r/%f&line=%l
+
+== Configuring: Other Editors ==
+
+General instructions for configuring some other editors and environments can be
+found here:
+
+  http://wiki.nette.org/en/howto-editor-link