docs: document how to trigger the tests for Beats PR (#608) (#609)

* docs: document how to trigger the tests for Beats PR

* docs: typo

Author: mdelapenya

e2e/README.md

@@ -187,6 +187,24 @@ To do so:
 Here you have a video reproducing the same steps:
 ![](./regression-testing.gif)
 
+### Running tests for a Beats pull request
+Because we trigger the E2E tests for each Beats PR that is packaged, it's possible to manually trigger it using CI user interface. To achieve it we must navigate to Jenkins and run the tests in the specific branch the original Beats PR is targeting.
+
+>For further information about packaging Beats, please read [Beat's CI docs](https://github.com/elastic/beats/blob/1de27eed058dd074b58c71094c7678b3536251cb/README.md#ci).
+
+To do so:
+
+1. Navigate to Jenkins: https://beats-ci.elastic.co/job/e2e-tests/job/e2e-testing-mbp/
+1. Login as a user
+1. Select the base branch for the test code: 7.10.x, 7.11.x, 7.x or master.
+1. In the left menu, click on `Buid with Parameters`.
+1. In the input parameters form, set the Beat version **for Fleet and Metricbeat** with the ID of your pull request, using `pr-` as prefix. I.e. `pr-23456`. 
+1. In the input parameters form, keep the stack version (for Fleet and Metricbeat) as is, to use each branch's default version.
+1. In the input parameters form, set the `GITHUB_CHECK_NAME` to `E2E Tests`. This value will appear as the label for the Github check for the E2E tests.
+1. In the input parameters form, set the `GITHUB_CHECK_REPO` to `beats`.
+1. In the input parameters form, set the `GITHUB_CHECK_SHA1` to the `SHA1` of the last commit in your pull request. This value will allow us to modify the mergeable status of that commit with the Github check.
+1. Click the `Build` button at the bottom of the parameters form.
+
 ## Noticing the test framework
 
 To generate the notice files for this project: