Add basic documentation

src/extern-sync/README.md

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+# Extern-Sync
+
+## 🎯 Purpose
+
+*Extern-Sync* is a .NET application to sync selected drilling data between a source and a target database. It consists of a couple of *sync tasks* that are executed in sequence.
+
+The application is packed into a Docker container and can be run in a containerized environment. The container exits with a non-zero exit code if any of the *sync tasks* fails.
+
+## 🛠️ Configuration
+
+The application is configured using environment variables. The following environment variables are mandatory:
+
+- `CONNECTIONSTRINGS__SourceBdmsContext`: The connection string to the source database.
+- `CONNECTIONSTRINGS__TargetBdmsContext`: The connection string to the target database.
+
+## 🧪 Unit Tests
+
+According to your needs, *sync tasks* can either be executed in a *in-memory* or a real PostgreSQL database using [Testcontainers](https://testcontainers.com/modules/postgresql/).
+
+## 🚀 Usage
+
+A full integration test can be run by executing the following commands:
+
+```bash
+# Setup environment (source and target databases)
+docker compose -f docker-compose.services.yml up --wait
+
+# Start the extern-sync container
+docker compose down && docker compose up --build
+```