Clarify installation procedure (#183)

* Add note regarding the externally-managed-environment error

* Provide commands that we use in tutorials.

---------

Co-authored-by: Benjamin Rodenberg <[email protected]>

README.md

@@ -27,9 +27,18 @@ preCICE-adapter for the open source computing platform FEniCS.
 It is recommended to install [fenicsprecice from PyPI](https://pypi.org/project/fenicsprecice/) via
 
 ```bash
-pip3 install --user fenicsprecice
+pip3 install fenicsprecice
 ```
 
+For more recent pip versions you may encounter the error `error: externally-managed-environment` during installation of the fenicsprecice. You can read why [here](https://packaging.python.org/en/latest/specifications/externally-managed-environments/). In this case, it is recommended to create a virtual environment and install the package in the virtual environment. The following commands will create the virtual environment `venv_name` at the location `path/of/your/venv/`:
+
+```bash
+python3 -m venv --system-site-packages path/of/your/venv/venv_name
+. path/of/your/venv/venv_name/bin/activate
+```
+
+You can replace `path/of/your/venv/venv_name` with the location and name you want to use for your virtual environment. The command `--system-size-packages` will allow the virtual environment to access the FEniCS installation on your system. Refer to the [Python documentation](https://docs.python.org/3/library/venv.html) for further details on virtual environments.
+
 This should work out of the box, if all dependencies are installed correctly. If you face problems during installation or you want to run the tests, see below for a list of dependencies and alternative installation procedures
 
 ### Clone this repository and use pip3