docs(readme): add documentation regarding salt.formula usage [skip ci]

* docs(readme): add documentation regarding salt.formula usage.

* docs(readme): change salt.formulas documentation to use double backtick.

Co-authored-by: Imran Iqbal <myii@users.noreply.github.com>

docs/README.rst

 
 Clone selected `Salt formulas
 <http://docs.saltstack.com/en/latest/topics/development/conventions/formulas.html>`_
-Git repositories under ``/srv/formulas`` and makes them available in the
-relevant ``file_roots`` settings. Pillar data can be used to customize all
-paths, URLs, etc.
+Git repositories under ``/srv/formulas`` and makes them available in the relevant ``file_roots`` settings. Please note that in order for ``file_roots`` to be updated, ``salt.master`` must be called after ``salt.formulas``. For example:
 
-Here's a minimal pillar sample installing two formulas in the base
-environment.
+::
+
+    base:
+      'saltmain':
+        - salt.formulas
+        - salt.master
+    
+
+Pillar data can be used to customize all paths, URLs, etc. Here's a minimal pillar sample installing two formulas in the base environment:
 
 ::