added instructions for

- image creation/modification
- python venv

docs/python-venv.md

@@ -0,0 +1,25 @@
+# How to work with python venv
+
+## create
+Create a virtual environment with:
+```
+    python -m venv <path/and/name/of/venv-dir>
+```
+**Example:**
+```
+    python -m venv .venv
+```
+The example command creates a directory `.venv` (in the current working directory) containing the created virtual environment.
+
+## use
+To use/activate the virtual environment, source `<venv-dir>/bin/activate`.
+**Example:**
+```
+    source .venv/bin/activate
+```
+or
+```
+    . .venv/bin/activate
+```
+
+To exit the virtual environment use the command/script `deactivate`.