added instructions for

- image creation/modification
- python venv

docs/create-image-file.md

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+
+1. download image
+1. extract image
+1. enlarge image with 'truncate'
+1. mount as loopback device
+1. modify image
+    1. modify partition table with fdisk
+        1. resize partition for rfs
+    1. resize filesystem
+    1. mount boot partition
+        1. make file 'ssh'
+        1. make user config
+        1. remove image resize from config file ~'cmdline.txt'
+    1. mount rootfs partition
+        1. do stuff

docs/python-venv.md

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+# 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`.