Add reference to from_qp_model (#1302)

I believe it is from section III of this paper but correct me if I'm
wrong @daniel-dudt : https://arxiv.org/pdf/2211.09829

desc/geometry/surface.py

@@ -350,6 +350,8 @@ def from_qp_model(
     ):
         """Create a surface from a near-axis model for quasi-poloidal symmetry.
 
+        Model is based off of section III of Goodman et. al. [1]_
+
         Parameters
         ----------
         major_radius : float
@@ -376,6 +378,11 @@ def from_qp_model(
         surface : FourierRZToroidalSurface
             Surface with given geometric properties.
 
+        References
+        ----------
+        .. [1] Goodman, Alan, et al. "Constructing Precisely Quasi-Isodynamic
+          Magnetic Fields." Journal of Plasma Physics 89.5 (2023): 905890504.
+
         """
         assert mirror_ratio <= 1
         a = major_radius * np.sqrt(elongation) / aspect_ratio  # major axis