add basic README.

README.md

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+# uptimerobot.com prometheus exporter
+
+Exports all your uptimerobot.com checks for prometheus scraping,
+so you can use external, third-party checks in your monitoring setup.
+
+*Caveat: This does not (yet) handle paging, so if you have a Pro account
+with uptimerobot.com you might have more checks defined than this script
+will handle.*
+
+## Requirements
+
+* Python
+* [requests](http://www.python-requests.org/en/master/)
+
+## Running
+
+Accepted parameters:
+
+* api_key: Your uptimerobot.com API key. See section 'API Settings' in [your account details](https://uptimerobot.com/dashboard#mySettings).
+* server_name (optional): Name to bind the HTTP server to. Default: 0.0.0.0
+* server_port (optional): Port to bind the HTTP server to. Default: 9705
+
+You can either pass script arguments (run `python exporter.py -h` for an explanation)
+or set the following environment variables:
+
+* `UPTIMEROBOT_API_KEY`
+* `UPTIMEROBOT_SERVER_NAME`
+* `UPTIMEROBOT_SERVER_PORT`
+
+## Docker
+
+Run `docker run -e 'UPTIMEROBOT_API_KEY=...' -p 9705:9705 --read-only hnrd/uptimerobot_exporter`.
+
+## docker-compose
+
+Example compose file:
+
+    version: '2.1'
+    
+    services:
+      exporter:
+        image: hnrd/uptimerobot_exporter
+        restart: unless-stopped
+        environment:
+          UPTIME_ROBOT_APIKEY: u123456-1a2b3c4d5e6f7890abc12323
+        ports:
+          - 9705:9705
+        read_only: true
+