Added elderly vaccination README

Author: AndrewC19

examples/covasim_/vaccinating_elderly/README.md

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+# Covasim Case Study: Prioritising Vaccination for the Elderly
+In this case study, we demonstrate how to use causal testing to determine whether prioritising the elderly
+has the expected effect on a series of vaccine-related outcomes. Specifically, cumulative infections, vaccines (number
+of doses administered), vaccinated (number of agents vaccinated), and maximum doses per agent. As explained in Section
+5.3 (Prioritising the elderly for vaccination), we expect that changing the Pfizer vaccine to additionally prioritise
+the elderly should cause vaccines and vaccinated to decrease (more restrictive policy), infections to increase (less
+are vaccinated), and no change to the maximum doses since this should always be 2.
+
+This case study directly executes Covasim under two input configurations that differ only in their vaccine input: one
+is the default Pfizer vaccine, the other is the same vaccine but additionally sub-targeting the elderly using a method 
+provided in the Covasim vaccine tutorial. We execute each of these input configurations 30 times and repeat this for
+four test cases: one focusing on each of the four previously mentioned outputs.
+
+Further details are provided in Section 5.3 (Prioritising the elderly for vaccination) of the paper.
+
+## How to run
+To run this case study:
+1. Ensure all project dependencies are installed by running `pip install .` from the top
+level of this directory (instructions are provided in the project README).
+2. Additionally, in order to run Covasim, install version 3.07 by running `pip install covasim==3.0.7`.
+3. Change directory to `causal_testing/examples/covasim_/vaccinating_elderly`.
+4. Run the command `python causal_test_vaccine.py`.
+
+This will run Covasim as described above and print out the causal test results for the effect of each input on each
+output.