Better docs on targeting nodes

docs/docs/guides.md

@@ -1,5 +1,39 @@
 # Guides
 
+## Target nodes spec
+
+You can choose whether a job is run on a node or nodes by specifying tags and a count of target nodes having this tag do you want a job to run.
+
+### Examples:
+
+Target all nodes with a tag:
+
+```
+{
+    "name": "job_name",
+    "command": "/bin/true",
+    "schedule": "@every 2m",
+    "tags": {
+        "role": "web"
+    }
+}
+```
+
+Target only two nodes of a group of nodes with a tag:
+
+```
+{
+    "name": "job_name",
+    "command": "/bin/true",
+    "schedule": "@every 2m",
+    "tags": {
+        "role": "web:2"
+    }
+}
+```
+
+Dkron will try to run the job in the amount of nodes indicated by that count having that tag.
+
 ## CRON Expression Format
 
 A cron expression represents a set of times, using 6 space-separated fields.