[doc] added documentation for pre-merge testing

[kuhnel]

Summary

Added pre-merge testing on Phabricator to the official LLVM documentation.

This is mostly a copy-and-paste from the existing documentation at https://github.com/google/llvm-premerge-checks

fixes https://github.com/google/llvm-premerge-checks/issues/275

[goncharov]

Thank you, Christian!

llvm/docs/Phabricator.rst

181–183

That was fixed in https://github.com/google/llvm-premerge-checks/issues/263 and can be removed.

[kuhnel]

resolved Mikhails comment

[• goncharov_del]

Test comment

llvm/docs/Phabricator.rst

 if you have a question, drop by on #llvm in IRC to get help.
 
 Note that as e-mail is the system of reference for code reviews, and some
 people prefer it over a web interface, we do not generate automated mail
 when a review changes state, for example by clicking "Accept Revision" in
 the web interface. Thus, please type LGTM into the comment box to accept
 a change from Phabricator.
 
+.. _pre-merge-testing:
+
+Pre-merge testing
+-----------------
+
+The pre-merge tests are a continuous integration (CI) workflow. The workflow 
+checks the patches uploaded to Phabricator before a user merges them to the main 
+branch - thus the term *pre-merge testing*. 
+
+When a user uploads a patch to Phabricator, Phabricator triggers the checks and
+then displays the results. This way bugs in a patch are contained during the 
+code review stage and do not pollute the main branch. 
+
+If you notice issues or have an idea on how to improve pre-merge checks, please 
+`create a new issue <https://github.com/google/llvm-premerge-checks/issues/new>`_ 
+or give a ❤️ to an existing one.
+
+Requirements
+^^^^^^^^^^^^
+
+To get a patch on Phabricator tested, the build server must be able to apply the
+patch to the checked out git repository. Please make sure that either:
+
+* You set a git hash as ``sourceControlBaseRevision`` in Phabricator which is
+  available on the GitHub repository, 
+* **or** you define the dependencies of your patch in Phabricator, 
+* **or** your patch can be applied to the main branch.
+
+Only then can the build server apply the patch locally and run the builds and
+tests.
+
+Accessing build results
+^^^^^^^^^^^^^^^^^^^^^^^
+Phabricator will automatically trigger a build for every new patch you upload or
+modify. Phabricator shows the build results at the top of the entry. Clicking on 
+the links (in the red box) will show more details:
+
+  .. image:: Phabricator_premerge_results.png
+
+The CI will compile and run tests, run clang-format and clang-tidy on lines
+changed.
+
+If a unit test failed, this is shown below the build status. You can also expand
+the unit test to see the details:
+
+  .. image:: Phabricator_premerge_unit_tests.png
+
 Committing a change
 -------------------
 
 Once a patch has been reviewed and approved on Phabricator it can then be
 committed to trunk. If you do not have commit access, someone has to
 commit the change for you (with attribution). It is sufficient to add
 a comment to the approved review indicating you cannot commit the patch
 yourself. If you have commit access, there are multiple workflows to commit the