add README & example

Signed-off-by: Lunik <lunik@tiwabbit.fr>

README.md

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+# Prometheus export for Gitea
+
+## What does it do ?
+
+Export repositories metrics with prometheus format.
+
+### Metrics exported
+
+| Metric             | Labels                                  | Type  |
+|:-------------------|:----------------------------------------|:------|
+| user               | ['username', 'full_name']               | Gauge |
+| org                | ['username', 'full_name', 'visibility'] | Gauge |
+| repo               | ['name', 'owner']                       | Gauge |
+| repo_size          | ['name', 'owner', 'unit']               | Gauge |
+| repo_empty         | ['name', 'owner']                       | Gauge |
+| repo_archived      | ['name', 'owner']                       | Gauge |
+| repo_stars         | ['name', 'owner']                       | Gauge |
+| repo_commits       | ['name', 'owner']                       | Gauge |
+| repo_branches      | ['name', 'owner']                       | Gauge |
+| repo_forks         | ['name', 'owner']                       | Gauge |
+| repo_issues        | ['name', 'owner']                       | Gauge |
+| repo_pull_requests | ['name', 'owner']                       | Gauge |
+
+## Build it
+
+```shell
+$ docker build -t gitea-exporter:latest .
+```
+
+## Run it
+
+Take a look at the [Docker compose file](docker-compose.yml)
+
+## Sources
+
+[Gitea](https://github.com/go-gitea/gitea)
+
+Thanks to [Langenfeld/py-gitea](https://github.com/Langenfeld/py-gitea)
+
+## Disclaimer
+
+This software was built by me, for my usages. It may not work "as it is" in your setup.

docker-compose.yml

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+version: '3.7'
+
+services:
+  gitea:
+    image: "gitea/gitea"
+    environment:
+      DB_TYPE: postgres
+      DB_HOST: "gitea_database:5432"
+      DB_NAME: gitea
+      DB_USER: gitea
+      DB_PASSWD: "changeit"
+    networks:
+      - gitea
+    volumes:
+      - /app/gitea/data:/data
+    depends_on:
+      - gitea_database
+
+  gitea_database:
+    image: "postgres"
+    environment:
+      POSTGRES_USER: gitea
+      POSTGRES_PASSWORD: "changeit"
+      POSTGRES_DB: gitea
+    networks:
+      - gitea
+    volumes:
+      - /app/gitea/database:/var/lib/postgresql/data
+
+  gitea_exporter:
+    image: "gitea-exporter:latest"
+    configs:
+      - source: gitea_exporter
+        target: /app/config.yml
+    networks:
+      - gitea
+
+configs:
+  gitea_exporter:
+    name: gitea-exporter-config
+    file: ./config.yml
+
+networks:
+  gitea: