Add documentation for SdfLib plugin.

Fixes #2182.

PiperOrigin-RevId: 690980373
Change-Id: Ifc94f928e9cfbdbef3cb6f11ad4bffed542bf49a

plugin/sdf/README.md

@@ -62,6 +62,16 @@ Parameters:
   - `radius1` [m]: major radius (default `0.35`).
   - `radius1` [m]: minor radius (default `0.15`).
 
+### SdfLib
+
+Implemented in [sdflib.cc](sdflib.cc). Example usage in [cow.xml](../../model/plugin/sdf/cow.xml).
+
+This plugin uses the library [SdfLib](https://github.com/UPC-ViRVIG/SdfLib) to compute a voxel-based approximation of a
+user-specified mesh. The mesh can be arbitrary and not necessarily convex. This offers an alternative to
+convex-decomposed meshes. The performance is likely to be slower than that of analytic SDFs, since a cubic
+approximation has to be evaluated on the convex grid. However, the SDF generation is done automatically, simplifying the
+task of creating an SDF, which can be difficult for complex shapes.
+
 ### How to make your own SDF
 
 Create your `MySDF.h` and `MySDF.cc` files in the SDF folder, where this README is located. Implement your SDF using the
@@ -98,3 +108,10 @@ class MySDF {
   MySDF(const mjModel* m, mjData* d, int instance);
 };
 ```
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