Add foundational Michelsen and STIFF3-vs-Gear references to README method section (#55)

* docs: add Michelsen and Weimer-Clough references in README

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* docs: normalize citation wording in README references

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README.md

@@ -174,8 +174,20 @@ The specific three-stage semi-implicit Runge-Kutta method (SIRK3) used by `stiff
 
 > Caillaud, J. B., & Padmanabhan, L. (1971). An improved semi-implicit Runge-Kutta method for stiff systems. *The Chemical Engineering Journal*, 2(4), 227–232. https://doi.org/10.1016/0300-9467(71)85001-3
 
+Related references by Michelsen that predate the Villadsen & Michelsen (1978) textbook include:
+
+> Michelsen, M. L. (1976). An efficient general purpose method for the integration of stiff ordinary differential equations. *AIChE Journal*, 22(3), 594–597. https://doi.org/10.1002/aic.690220327
+>
+> Michelsen, M. L. (1977). Application of semi-implicit runge—kutta methods for integration of ordinary and partial differential equations. *The Chemical Engineering Journal*, 14(2), 107–112. https://doi.org/10.1016/0300-9467(77)85005-3
+
+These papers precede the textbook, but the original source code refactored in this repository was taken from Villadsen & Michelsen (1978), as noted above.
+
 The adaptive stepsize selection strategy is described in Villadsen & Michelsen (1978), Section 8.2.3, pages 314–317.
 
+A comparison of STIFF3 with Gear's method (Hindmarsh and Gear) was published in:
+
+> Weimer, A. W., & Clough, D. E. (1979). A critical evaluation of the semi-implicit Runge‐Kutta methods for stiff systems. *AIChE Journal*, 25(4), 730–732. https://doi.org/10.1002/aic.690250424
+
 The two tolerance parameters `eps` and `w` together control how accurately the solution is tracked:
 
 - **`eps`** — a single scalar that sets the overall accuracy goal. A smaller value requests a more accurate solution at the cost of more integration steps. Typical values range from `1e-3` (low accuracy) to `1e-8` (high accuracy).