samples: add info about ShaderTerrainMesh optimization config vars

(Cherry-pick from master)

[skip ci]

samples/shader-terrain/main.py

@@ -21,6 +21,14 @@ class ShaderTerrainDemo(ShowBase):
             gl-coordinate-system default
             window-title Panda3D ShaderTerrainMesh Demo
             filled-wireframe-apply-shader true
+
+            # As an optimization, set this to the maximum number of cameras
+            # or lights that will be rendering the terrain at any given time.
+            stm-max-views 8
+
+            # Further optimize the performance by reducing this to the max
+            # number of chunks that will be visible at any given time.
+            stm-max-chunk-count 2048
         """)
 
         # Initialize the showbase