[gnome-builder] doc: add some editor docs

[Christian Hergert <chergert src gnome org>]

commit 43efb630fdaea4018e549427e7e9d2292b44bd59
Author: Christian Hergert <chergert redhat com>
Date:   Sat Sep 2 22:17:48 2017 -0700

    doc: add some editor docs

 doc/exploring.rst                    |   28 ++++++++++++++++++++++++++++
 doc/figures/autocompletion.png       |  Bin 0 -> 185671 bytes
 doc/figures/inline-documentation.png |  Bin 0 -> 213982 bytes
 3 files changed, 28 insertions(+), 0 deletions(-)
---
diff --git a/doc/exploring.rst b/doc/exploring.rst
index 9d730fe..a7e1380 100644
--- a/doc/exploring.rst
+++ b/doc/exploring.rst
@@ -108,6 +108,34 @@ Clicking the cancel button will abort the current build.
    :align: center
 
 
+Editor
+======
+
+When Builder opens your project, it will place you in the editor perspective.
+This is where you develop your project.
+
+Autocompletion
+--------------
+
+Builder has built-in support for various autocompletion engines.
+Start typing to get word suggestions.
+
+.. image:: figures/autocompletion.png
+   :width: 960 px
+   :align: center
+
+Documentation
+-------------
+
+If you hover the pointer over API that Builder knows about, it can show you the documentation.
+You can also use ``F2`` to bring up the documentation with your insertion cursor on the word.
+Use ``Shift+K`` if you're using Vim keybindings.
+
+.. image:: figures/inline-documentation.png
+   :width: 1024 px
+   :align: center
+
+
 Preferences
 ===========
 
diff --git a/doc/figures/autocompletion.png b/doc/figures/autocompletion.png
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diff --git a/doc/figures/inline-documentation.png b/doc/figures/inline-documentation.png
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index 0000000..0dd3204
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