Document installability test

Signed-off-by: Nikola Forró <nforro@redhat.com>

Author: nforro

fedora-ci/jobs.md

@@ -11,7 +11,8 @@ Scratch builds in [Koji](https://koji.fedoraproject.org/koji/) built from SCM.
 
 ### Installability
 
-TBD (The work is in progress.)
+[Installability test](https://github.com/fedora-ci/installability-pipeline)
+in [Testing Farm](https://testing-farm.io) run with a successfull scratch build.
 
 ### Custom test plans
 

fedora-ci/retriggering.md

@@ -14,3 +14,22 @@ To retrigger a scratch build, add the following comment to the pull request:
 ```
 
 Only packagers can retrigger scratch builds.
+
+## Tests
+
+To retrigger all tests, add the following comment to the pull request:
+
+```
+/packit-ci test
+```
+
+To retrigger a specific test, you can add the test identifier to the command:
+
+```
+/packit-ci test installability
+```
+
+`installability` is the only supported identifier so far and it represents
+the [Installability](fedora-ci/jobs#installability) test.
+
+Only packagers can retrigger tests.