Added LR91 README

Author: AndrewC19

examples/lr91/README.md

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+# Luo-Rudy 1991 Case Study: APD90 Sensitivity Analysis
+In this case study, we demonstrate how causal testing can be used to implement a sensitivity analysis approach measuring the
+extent to which a series of conductance-based inputs effect a single output, APD90, in the seminal Luo-Rudy 1991
+cardiac action potential model. As described in Section 5.2 of the paper, this involves running a series of causal test
+cases that incrementally increase/decrease the treatment value for each input above/below its mean to quantify how much
+a given change input causes a change in APD90.
+
+Here we expect that increasing G_K, G_b, and G_K1, will cause APD90 to decrease, increasing G_si will
+cause APD90 to increase, and increasing G_Na and G_Kp to have no significant effect on APD90. Further details
+can be found in Section 5.2 of the paper.
+
+## How to run
+To run this case study:
+1. Ensure all project dependencies are installed by running `pip install .` in the top level directory
+   (instructions are provided in the project README).
+2. Change directory to `causal_testing/examples/lr91`.
+3. Run the command `python causal_test_max_conductances.py`
+
+This should print a series of causal test results covering the effects of a range of different sized interventions made
+to the inputs on APD90, and should also plot Figure 2 from the paper.