Add autoscaler docs

docs/docs/30-administration/80-autoscaler.md

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+# Autoscaler
+
+If your would like dynamically scale your agents with the load, you can use [our autoscaler](https://github.com/woodpecker-ci/autoscaler).
+
+Please note that the autoscaler is not feature-complete yet. You can follow the progress [here](https://github.com/woodpecker-ci/autoscaler#roadmap).
+
+## Setup
+
+### docker-compose
+
+If you are using docker-compose you can add the following to your docker-compose.yml file:
+
+```yaml
+version: '3'
+
+services:
+  woodpecker-server:
+    image: woodpeckerci/woodpecker-server:next
+    [...]
+
+  woodpecker-autoscaler:
+    image: woodpeckerci/autoscaler:next
+    restart: always
+    depends_on:
+      - woodpecker-server
+    environment:
+      - WOODPECKER_SERVER=https://your-woodpecker-server.tld # the url of your woodpecker server / could also be a public url
+      - WOODPECKER_TOKEN=${WOODPECKER_TOKEN} # the api token you can get from the UI https://your-woodpecker-server.tld/user
+      - WOODPECKER_MIN_AGENTS=0
+      - WOODPECKER_MAX_AGENTS=3
+      - WOODPECKER_WORKFLOWS_PER_AGENT=2 # the number of workflows each agent can run at the same time
+      - WOODEPCKER_GRPC_ADDR=https://grpc.your-woodpecker-server.tld # the grpc address of your woodpecker server, publicly accessible from the agents
+      - WOODEPCKER_GRPC_SECURE=true
+      - WOODPECKER_AGENT_ENV= # optional environment variables to pass to the agents
+      - WOODPECKER_PROVIDER=hetznercloud # set the provider, you can find all the available ones down below
+      - WOODPECKER_HETZNERCLOUD_API_TOKEN=${WOODPECKER_HETZNERCLOUD_API_TOKEN} # your api token for the Hetzner cloud
+```