Document OpenJPEG and libfreetype requirements

CONTRIBUTING.md

@@ -5,13 +5,21 @@
 ### Requirements
 
 * Python 3.5+
-    * Windows: https://www.python.org/downloads
-    * Mac: `$ brew install python3`
-    * Linux: `python3.5` and `python3.5-dev` packages
+    - Windows: https://www.python.org/downloads
+    - macOS: `$ brew install python3`
+    - Linux: `python3.5` and `python3.5-dev` packages
 * Make:
-    * Windows: http://cygwin.com/install.html
-    * Mac: https://developer.apple.com/xcode
-    * Linux: http://www.gnu.org/software/make (likely already installed)
+    - Windows: http://cygwin.com/install.html
+    - macOS: https://developer.apple.com/xcode
+    - Linux: http://www.gnu.org/software/make (likely already installed)
+* OpenJPEG:
+    - Windows: ?
+    - macOS: `$ brew install openjpeg`
+    - Linux: ?
+* libfreetype:
+    - Windows: ?
+    - macOS: `$ brew install freetype`
+    - Linux: ?
 
 ### Installation