[Doc] Add reference for PLOG formulation

doc/doxygen/cantera.bib

@@ -79,6 +79,12 @@
         Environment},
     url = {https://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote1402.pdf},
     year = {1993}}
+@misc{gou2011,
+    author = {X.~Gou and J.~A.~Miller and W.~Sun and Y.~Ju},
+    title = {Implementation of {PLOG} function in {C}hemkin {II} and {III}},
+    howpublished = {\url{https://engine.princeton.edu/model-reduction/}},
+    note = {Accessed: 2022-04-25},
+    year = {2011}}
 @article{gudmundsson2001,
     author = {J.~T.~Gudmundsson},
     journal = {Plasma Sources Science and Technology},

doc/sphinx/reference/kinetics/rate-constants.md

@@ -157,8 +157,8 @@ Chemically-activated reactions can be defined in the YAML format using the
 ## Pressure-Dependent Arrhenius Rate Expressions (P-Log)
 
 This parameterization represents pressure-dependent reaction rates by logarithmically
-interpolating between Arrhenius rate expressions at various pressures. Given two rate
-expressions at two specific pressures:
+interpolating between Arrhenius rate expressions at various pressures {cite:p}`gou2011`.
+Given two rate expressions at two specific pressures:
 
 $$
 P_1: k_1(T) = A_1 T^{b_1} e^{-E_1 / RT}

include/cantera/kinetics/PlogRate.h

@@ -69,8 +69,8 @@ protected:
  * @f]
  * Multiple rate expressions may be given at the same pressure, in which case
  * the rate used in the interpolation formula is the sum of all the rates given
- * at that pressure. For pressures outside the given range, the rate expression
- * at the nearest pressure is used.
+ * at that pressure @cite gou2011. For pressures outside the given range, the
+ * rate expression at the nearest pressure is used.
  * @ingroup otherRateGroup
  */
 class PlogRate final : public ReactionRate