Add README with setup and usage details

README.md

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+# fmriprep_stats
+
+This repository contains utilities to download usage statistics of fMRIPrep from Sentry.io.
+
+## Setup
+
+1. Install Python 3.8 or newer.
+2. Install dependencies using `requirements.txt`:
+
+   ```bash
+   python -m pip install -r requirements.txt
+   ```
+
+## Running `src/run.py`
+
+`src/run.py` exposes a command line interface built with Click. Before running,
+set a `SENTRY_TOKEN` environment variable with a valid token.
+
+Example usage:
+
+```bash
+SENTRY_TOKEN=your_token python src/run.py get -m started -m success -s 2023-01-01 -u 2023-01-31
+```
+
+This will fetch the "started" and "success" events for January 2023. Omit the
+`-s` and `-u` options to use the default window (last 90 days). Run
+
+```bash
+python src/run.py --help
+```
+
+for a description of all available options.