Modifications to humanoid model.

- Increased hip flexion maximum.
- Symmetrised shoulder and ankle joints.
- Added hamstring tendons which couple the hip and knee at large flexion values.
- Moved duplicated values into defaults.
- Added two keyframes.
- Improved lighting.
- Changed naming convention.
- Moved description into README.md

PiperOrigin-RevId: 473745371
Change-Id: I95bf915c4caf3d5cf06c325cd73284d78a3d836e

model/humanoid/README.md

@@ -0,0 +1,17 @@
+Humanoid
+========
+
+Degrees of Freedom: 27
+Actuators: 21
+
+This simplified humanoid model, introduced in [1], is designed for bipedal locomotion
+behaviours. While several variants of it exist in the wild, this version is based on the model
+in the DeepMind Control Suite [2], which has fairly realistic actuator gains.
+
+[1] [Synthesis and Stabilization of Complex Behaviors through Online Trajectory Optimization]
+     (https://doi.org/10.1109/IROS.2012.6386025).
+
+[2] [DeepMind Control Suite](https://arxiv.org/abs/1801.00690).
+
+
+![humanoid](humanoid.png)