tags explained [CI skip]

README.md

@@ -3,3 +3,38 @@ This is a project to get started on the gitlab CI for compiling and testing code
 
 Important links:
 -  Explanations of the YAML syntax of the file **.gitlab-ci.yml** [found here](https://docs.gitlab.com/ce/ci/yaml)
+
+
+## Tags:
+Tags only make sence if there are multiple runners. 
+They allow to choose on which runner the job will be executed.
+
+**You don't need tags if you have only one runner!**
+
+The tags are used to select a specific runner. Note the following:
+1. Each job is assigned to only **one** runner!
+1. If job has no tag
+    - one runner must be configured to run untagged jobs 
+1. Job has tags
+    - **all** tags of a job **must match** a subset of the tags of **one** runner
+
+Each job can have one or multiple tags, here an example:
+```
+job_1:
+  stage: build
+  tags:
+    - tag1_runnerA
+job_2:
+  stage: build
+  tags:
+    - tag1_runnerB
+job_3:
+  stage: build
+  tags:
+    - tag1_runnerA
+    - tag2_runnerA
+```
+- runner A  has the tags `tag1_runnerA` and `tag2_runnerA` ... and therefore runs  `job_1` and `job_3`
+- runner B  has the tags `tag1_runnerB` and `tag2_runnerB` ... and therefore runs only `job_2`
+- **Careful**, the tag list should contain only tags of **one** runner!
+- **Careful**, different runners cannot have the same tag!