doc: update setup with poetry

Author: patrit

docs/setup.md

@@ -13,7 +13,34 @@ Start the CLI and the print the help page with:
 docker run --rm -it baloise/gitopscli --help
 ```
 
-## From Source
+## From Source With Virtualenv
+
+Use this for developement and if you want to prevent dependency clashes with other programs in a user installation.
+
+Clone the repository and install the GitOps CLI on your machine:
+```bash
+git clone https://github.com/baloise/gitopscli.git
+cd gitopscli/
+poetry install
+```
+You can now use it from the command line:
+```bash
+poetry run gitopscli --help
+```
+If you don't need the CLI anymore, you can uninstall it with
+```bash
+poetry env remove --all
+```
+
+Note: if your poetry is not up to date to handle the files you can use a locally updated version.
+Execute the following command in your cloned gitopscli directory to use an updated poetry without changing your system installation:
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+pip3 install poetry # installs it in the venv
+```
+
+## From Source Into User Installation
 
 Clone the repository and install the GitOps CLI on your machine:
 ```bash