Add workaround note for OpenGL issues

README.md

@@ -30,6 +30,13 @@ from fairmotion.data import bvh
 BVH_FILENAME = “PATH_TO_BVH_FILE”
 motion = bvh.load(BVH_FILENAME)
 ```
+
+If you recieve errors like the ones below, you can find the workaround [here](https://stackoverflow.com/questions/65202395/pyopengl-on-macos-bigsur-and-opengl-error-nullfunctionerror#:~:text=if%20name%20%3D%3D%20%27OpenGL%27%3A%0A%20%20%20%20fullName%20%3D%20%27/System/Library/Frameworks/OpenGL.framework/OpenGL%27%0Aelif%20name%20%3D%3D%20%27GLUT%27%3A%0A%20%20%20%20fullName%20%3D%20%27/System/Library/Frameworks/GLUT.framework/GLUT%27).
+```
+ImportError: ('Unable to load OpenGL library', 'dlopen(OpenGL, 10): image not found', 'OpenGL', None)
+or
+OpenGL.error.NullFunctionError: Attempt to call an undefined function glutInit, check for bool(glutInit) before calling
+```
 ### Motion manipulation
 
 The motion object can be manipulated in both modular and matrix forms. Here, we translate the object to a fixed global position `[1, 1, 1]` and select a time slice from frame `20` to frame `30`.