Document how to create the virtual environment

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -9,9 +9,19 @@ The figure shows the trajectory in the plane, the secular function `f(mu)`, the
 `||x0 - x(mu)||` and `f'(mu)`, together with the KKT points: `x(mu*)`, the other roots of `f`
 and, when `x0` has a zero normalized coordinate, the degenerate points `x^d`.
 
+## Setup
+
+Create a virtual environment in `venv/` (ignored by git) and install the dependencies
+(tested with Python 3.13):
+
+    python3 -m venv venv
+    source venv/bin/activate
+    pip install -r requirements.txt
+
+Run `source venv/bin/activate` again in each new shell, and `deactivate` to leave the environment.
+
 ## Usage
 
-    pip install -r requirements.txt
     python plot_mu_trajectories.py                         # every case, into figures/
     python plot_mu_trajectories.py x_trajectories_mu_2     # a single case
     pytest