Added agent registration docs (#1921)

---------

Co-authored-by: 6543 <6543@obermui.de>

docs/docs/30-administration/15-agent-config.md

@@ -2,7 +2,7 @@
 
 Agents are configured by the command line or environment variables. At the minimum you need the following information:
 
-```yaml
+```diff
 # docker-compose.yml
 version: '3'
 
@@ -23,7 +23,7 @@ The following are automatically set and can be overridden:
 
 By default the maximum processes that are run per agent is 1. If required you can add `WOODPECKER_MAX_WORKFLOWS` to increase your parallel processing on a per-agent basis.
 
-```yaml
+```diff
 # docker-compose.yml
 version: '3'
 
@@ -36,6 +36,39 @@ services:
 +   - WOODPECKER_MAX_WORKFLOWS=4
 ```
 
+## Agent registration on server
+
+When the agent starts, it connects to the server using token from `WOODPECKER_AGENT_SECRET`. The server identifies agent and, if such agent doesn't exist, register him.
+There are two types of token, so would be two ways of agent registration.
+
+### Using system token
+
+_System token_ is a token that is used system-wide, e. g. when you set the same token in `WOODPECKER_AGENT_SECRET` on both the server and the agents.
+
+In that case registration process would be as follows:
+
+1. First time Agent communicates with Server using system token;
+2. Server registers Agent in DB, generates ID and sends this ID back to Agent;
+3. Agent stores ID in a file configured by `WOODPECKER_AGENT_ID_FILE`.
+
+At the following startups Agent uses system token **and** ID.
+
+### Using agent token
+
+_Agent token_ is a token that is used by only particular agent. This unique token also configured by `WOODPECKER_AGENT_SECRET`, but only on the agent side.
+
+In that case you probably doesn't configure `WOODPECKER_AGENT_SECRET` on the server side. The registration process would be as follows:
+
+1. Administrator registers Agent manually in _Server settings - Agents - Add agent_;
+![Agent creation](./new-agent-registration.png)
+![Agent created](./new-agent-created.png)
+2. The token generated in previous step have to be provided to Agent in `WOODPECKER_AGENT_SECRET`;
+3. First time Agent communicates with Server using agent token;
+4. Server identifies Agent by the token and fills additional information provided by Agent;
+![Agent connected](./new-agent-connected.png)
+
+At following startups Agent uses own token only.
+
 ## All agent configuration options
 
 Here is the full list of configuration options and their default variables.