Add 3D video tutorials (#212)

* Add JamesTKhan tutorial series

* Fix heading hierarchy

* Normalise this Markdown style a little

Author: Frosty-J

wiki/articles/external-tutorials.md

@@ -6,7 +6,7 @@ This page gives a list of some **external tutorials**, if our wiki here isn't en
 *Preliminary note:* Given the nature of this page's content, it is likely that some of the links displayed here might become offline or even change. If you spot some broken link or wrong information please feel free to edit this page so we can keep our wiki as up-to-date as possible. Thank you.
 {: .notice--info}
 
-# General tutorial websites
+## General tutorial websites
 These are webpages where individuals or companies write about their experience with libGDX from a technical perspective. Some of the tutorials present in these pages are also referenced below in more topic specific sections but if you're just looking to some websites where people talk about their general experiences with libGDX you should start here.
 
 ### Some one-page tutorials for beginners
@@ -26,13 +26,18 @@ These are webpages where individuals or companies write about their experience w
 - [Asteroid game – by ForeignGuyMike](https://www.youtube.com/playlist?list=PL-2t7SM0vDfeZUKeM7Jm4U9utHwFS1N-C) (2014)
 - [By dermetfan – 25 parts](https://www.youtube.com/playlist?list=PLXY8okVWvwZ0JOwHiH1TntAdq-UDPnC2L) (2013)
 
-# Topic-specific tutorials
+## Topic-specific tutorials
+
+### 3D Graphics
+* [Video series by JamesTKhan](https://www.youtube.com/playlist?list=PLjUR2MkQ0cuHZ70Ps8F9WMyoyKHKAbYvQ), covering [gdx-gltf](https://github.com/mgsx-dev/gdx-gltf), player/camera movement, and terrain
+  * [Accompanying GitHub repository](https://github.com/JamesTKhan/gdx-gltf-starter/commits/video_7_terrain_collision/) 
+
 ### AdMob integration for libGDX Android projects
 * [norakomi.com](https://web.archive.org/web/20200807000743/http://norakomi.com/tutorial_admob_introduction.php)
 
 ### Lighting
- * [Normal Mapping](https://web.archive.org/web/20200508234246/http://www.java-gaming.org/topics/glsl-using-normal-maps-to-illuminate-a-2d-texture-libgdx/27516/view.html)
- * [Shadow Mapping](https://web.archive.org/web/20200429061153/https://www.microbasic.net/tutorials/shadow-mapping/Full.html)
+* [Normal Mapping](https://web.archive.org/web/20200508234246/http://www.java-gaming.org/topics/glsl-using-normal-maps-to-illuminate-a-2d-texture-libgdx/27516/view.html)
+* [Shadow Mapping](https://web.archive.org/web/20200429061153/https://www.microbasic.net/tutorials/shadow-mapping/Full.html)
 
 ### UI with Scene2D
 * [From the Ground Up Series by raeleus](https://github.com/raeleus/skin-composer/wiki/From-the-Ground-Up:-Scene2D.UI-Tutorials) (recommended!)
@@ -55,7 +60,7 @@ These are webpages where individuals or companies write about their experience w
 * [Video tutorial series by dermetfan](https://www.youtube.com/playlist?list=PLXY8okVWvwZ0qmqSBhOtqYRjzWtUCWylb)
    * [gdx-tiled-preprocessor video tutorial](https://www.youtube.com/watch?v=q5D-TzlCRPM) (Warning: the gdx-tiled-preprocessor was merged into gdx-tools)
 
-# Some simple open-source projects for reference
+## Some simple open-source projects for reference
 These are some open-source projects found on the web that use libGDX and can be used as reference. This list is intentionally kept as clean and interesting as possible (you won't see repeated projects nor _Hello-World kind-of-games_ here).
 * [Fire and Ice](https://github.com/lyze237/FireAndIce) (uses tile maps)
 * [On Guard](https://github.com/lyze237/On-Guard) (uses tile maps)