docs: adds documentation for comupute backends

* Addresses reanahub/reana-job-controller/issues/71

docs/userguide.rst

@@ -412,7 +412,8 @@ Declare necessary resources
 You can declare other additional runtime dependencies that your workflow needs
 for successful operation.
 
-**CVMFS**
+CVMFS
+~~~~~
 
 If your workflow needs to access `CVMFS <https://cernvm.cern.ch/portal/filesystem>`_
 filesystem, you should provide a ``cvmfs`` sub-clause of the ``resources`` clause that
@@ -433,7 +434,8 @@ For example:
             - ls -l /cvmfs/fcc.cern.ch/sw/views/releases/
 
 
-**Kerberos**
+Kerberos
+~~~~~~~~
 
 If your workflow requires Kerberos authentication, you should add ``kerberos: true``
 for the steps in need. Please note that step's docker image
@@ -494,6 +496,79 @@ Yadage example:
         resources:
           - kerberos: true
 
+Compute backends
+~~~~~~~~~~~~~~~~
+
+REANA supports Kubernetes as a primary compute backend alongside HTCondor and
+Slurm.
+
+In order to use HTCondor or Slurm clusters user should upload their CERN username
+and keytab secrets using:
+
+.. code-block:: console
+
+    $ reana-client secrets-add --env CERN_USER=johndoe
+                               --env CERN_KEYTAB=.keytab
+                               --file ~/.keytab
+
+ .. note::
+    Please note that CERN Slurm cluster access is not granted by
+    `default <https://batchdocs.web.cern.ch/linuxhpc/access.html>`_.
+
+**Kubernetes**
+
+Kubernetes is a default REANA job compute backend. If `step` does not contain
+`compute_backend` specification, it will be executed on the default one.
+
+.. code-block::
+
+   # Serial example
+   ...
+   steps:
+      - name: reana_demo_helloworld_htcondorcern
+        environment: 'python:2.7-slim'
+        compute_backend: kubernetes
+        commands:
+            - python "${helloworld}"
+   ...
+
+**HTCondor**
+
+In order to execute the job on the HTCondor cluster user must specify ``htcondorcern``
+as the step's execution backend in the workflow specification.
+
+.. code-block::
+
+   # Serial example
+   ...
+   steps:
+      - name: reana_demo_helloworld_htcondorcern
+        environment: 'python:2.7-slim'
+        compute_backend: htcondorcern
+        commands:
+            - python "${helloworld}"
+   ...
+
+Examples for CWL and Yadage can be found in
+`REANA example - "hello world"<https://github.com/reanahub/reana-demo-helloworld>``
+
+**Slurm**
+
+In order to execute the job on the Slurm cluster user must specify ``slurmcern``
+as the step's execution backend in the workflow specification.
+
+.. code-block::
+
+   # Serial example
+   ...
+   steps:
+      - name: reana_demo_helloworld_htcondorcern
+        environment: 'python:2.7-slim'
+        compute_backend: slurmcern
+        commands:
+            - python "${helloworld}"
+   ...
+
 
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 --------------------------------