docs: document how requirements.txt is updated

we should update this document once
jazzband/pip-tools#625 is fixed and/or
jazzband/pip-tools#908 is merged

Signed-off-by: Kefu Chai <kchai@redhat.com>

docs/INSTALL.rst

@@ -60,3 +60,13 @@ Or from GitHub::
 
 where the dependencies for orchestrating are installed. They are used for
 interacting with the services to schedule tests and to report the test results.
+
+
+Update Dependencies
+-------------------
+
+We track the dependencies using ``requirements.txt``. These packages are tested,
+and should work with teuthology. But if you want to bump up the version of them,
+please use follow command to update this file ::
+
+  echo "-e .[coverage,orchestra,test]" | pip-compile - -qo- | sed '/^-e / d' > requirements.txt