Document how to add a new writer

c.f. #2899

docs/tips.rst

@@ -93,21 +93,21 @@ by the ``ghp-import`` command) to the ``elemoine.github.io`` repository's
 
     To publish your Pelican site as User Pages, feel free to adjust the
     ``github`` target of the Makefile.
-    
+
 Another option for publishing to User Pages is to generate the output files in
 the root directory of the project.
 
 For example, your main project folder is ``<username>.github.io`` and you can
 create the Pelican project in a subdirectory called ``Pelican``. Then from
 inside the ``Pelican`` folder you can run::
-    
+
     $ pelican content -o .. -s pelicanconf.py
 
 Now you can push the whole project ``<username>.github.io`` to the master
 branch of your GitHub repository::
-    
+
     $ git push origin master
-    
+
 (assuming origin is set to your remote repository).
 
 Custom 404 Pages
@@ -179,4 +179,4 @@ If you are using ``develop-server.sh``,  add this to the top::
 
 and modify the ``pelican.server`` line as follows::
 
-    $PY -m pelican.server $port --ssl --cert="$CERT" --key="$KEY" &
+    $PY -m pelican.server $port --ssl --cert="$CERT" --key="$KEY" &