Better describe python venv

Updated donation instructions for CPU usage on Linux/MacOS.

Author: melroy89

readme.md

@@ -20,12 +20,14 @@ A manual is available [online](https://cppcheck.sourceforge.io/manual.pdf).
 Cppcheck is a hobby project with limited resources. You can help us by donating CPU (1 core or as many as you like). It is simple:
 
  1. Download (and extract) Cppcheck source code.
- 2. Run:
+ 2. Run (Linux/MacOS example):
     ```
     cd cppcheck/
-    virtualenv .env
-    .env/bin/pip install -r tools/donate-cpu-requirements.txt
-    .env/bin/python tools/donate-cpu.py
+    python3 -m venv ~/.venv_cppcheck
+    source ~/.venv_cppcheck/bin/activate
+
+    pip install -r tools/donate-cpu-requirements.txt
+    ./tools/donate-cpu.py
     ```
 
 The script will analyse debian source code and upload the results to a cppcheck server. We need these results both to improve Cppcheck and to detect regressions.