Add .gitignore for flit to build the package

.gitignore

+*~
+*.pyc
+__pycache__
+
+*.csv

README.md

@@ -13,3 +13,27 @@ Packaging informations :
 - https://packaging.python.org/en/latest/tutorials/packaging-projects/
 - https://choosealicense.com/
 
+## Troubleshooting
+
+If we have untracked data files in the package tree, flist refuses to build the
+package.
+
+```shell
+$ flit build
+Fetching list of valid trove classifiers                                                                                                                                                    I-flit.validate
+Untracked or deleted files in the source directory. Commit, undo or ignore these files in your VCS.
+```
+
+If we add a line to ignore, e.g. all csv files, in `.gitignore` we get a warning when we want to add one.
+
+```shell
+$ git add src/import-resources-example/data/WPP2022_TotalPopulationBySex.csv
+Les chemins suivants sont ignorés par un de vos fichiers .gitignore :
+src/import-resources-example/data/WPP2022_TotalPopulationBySex.csv
+astuce: Utilisez -f si vous voulez vraiment les ajouter.
+astuce: Éliminez ce message en lançant
+astuce: "git config advice.addIgnoredFile false"
+```
+
+As we can append the data files with `-f`, I will ignore the data file in `.gitignore`.
+