duniterpy#169: Update pre-commit doc. Document CI linter hook usage

Give the choices to install pre-commit via the distrib or via pip

CONTRIBUTING.md

@@ -30,7 +30,7 @@ silkaj> cd ..
 git clone https://git.duniter.org/clients/python/duniterpy
 ```
 
-Use DuniterPy as a [path dependency](https://python-poetry.org/docs/versions/#path-dependencies):
+Use DuniterPy as a [path dependency](https://python-poetry.org/docs/dependency-specification/#path-dependencies):
 ```bash
 poetry add ../duniterpy
 ```
@@ -47,19 +47,45 @@ Please read their documentations on how to use them the best possible.
 ## Pre-commit
 We are using [`pre-commit`](https://pre-commit.com/) tool to perform checks on staged changes before committing.
 We are using it for `black` formatting, `isort` imports sorting, and `gitlab-ci` linting. We will use it for `pylint` code linting, `mypy` typing in the future.
-Depending whether you prefer to work ouside or inside Poetry virtual environment, you should stick to one usage or use both to still have this usefull checks.
-`pre-commit` can be used from the installation on your system or from inside Poetry virtual environment where it is installed.
 
-To install the `git-hooks`, run:
+Install `pre-commit` from your distribution. In case it is an outdated version, install it from `pip`:
 ```bash
 sudo apt install pre-commit
-silkaj> pre-commit install
+pip install --user pre-commit
 ```
 
+To install the `git-hooks`, run:
 ```bash
-silkaj> poetry shell
-(silkaj-58dUTebQ-py3.9) pre-commit install
+> pre-commit install
+```
+
+Then each time you commit changes, the hooks will perform checks.
+
+To manually run one of the tool above, run (eg for `isort`):
+```bash
+> pre-commit run --all-files isort
+```
+To run all checks on all files:
+```bash
+> pre-commit run -a
+```
+
+### Authorization for GitLab CI linter hook
+`pre-commit run -a (gitlab-ci-linter)` is failing due to authorization required for CI lint API accesses.
+When running this command, just ignore this failed hook.
+In case you want to commit a `.gitlab-ci.yml` edition, this hook will prevent the commit creation.
+You can [skip the hooks](https://stackoverflow.com/a/7230886) with `git commit -m "msg" --no-verify`.
+This is fine for occasional `.gitlab-ci.yml` editions. In case you would like to edit this file more often and have it checked, ask a maintainer to provide you with `GITLAB_PRIVATE_TOKEN` environment variable that can be set into a shell configuration.
+With Bash, in `$HOME/.bashrc` add the following:
+```bash
+export GITLAB_PRIVATE_TOKEN=""
+```
+
+With Fish, in `$HOME/.config/fish/config.fish` add the following:
+```fish
+set -xg GITLAB_PRIVATE_TOKEN ""
 ```
+Check out duniterpy#169 for more details.
 
 ### Black formatting
 We are using [Black](https://github.com/psf/black) formatter tool.