doc: add README

README.md

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+# Redmine CSV loader
+
+Create Redmine users from CSV and add users to a group.
+
+⚠️ This project is only a draft, it's working but not intended to be use widely ⚠️
+
+### Requirements
+
+The users CSV file must be in utf-8 using "," or ";" as separator. The required
+headers must be exactly "firstname", "lastname" and "mail" in any order (all
+other header will be skipped).
+
+```csv
+firstname;lastname;mail
+jean;dupont;jean.dupont@example.com
+alice;coger;alice.coger@example.com
+bob;parraut;bob.parraut@example.com
+```
+
+## Usage
+
+### Get help
+
+```sh
+./redmine-csv-loader --help
+```
+
+### Create users
+
+```sh
+./redmine-csv-loader ./path/to/csv -k "my_api_key"
+```
+
+### Add existing users to a group
+
+```sh
+./redmine-csv-loader ./path/to/csv -k "my_api_key" -a -g "my_group"
+```