Add command to the dev guide.

doc/internal/dev_start_guide.txt

@@ -70,6 +70,22 @@ then go to your fork's github page on the github website, select your feature
 branch and hit the "Pull Request" button in the top right corner.
 If you don't get any feedback, bug us on the theano-dev mailing list.
 
+When the your pull request have been merged, you can delete the branch
+from the github list of branch. That is usefull to don't have too many
+that stay there!
+
+.. code-block:: bash
+
+   git push origin :my_shiny_feature
+
+
+You can keep you local repo up to date with central/master with those commands:
+
+.. code-block:: bash
+
+   git checkout master
+   git fetch central
+   git merge central/master
 
 Cleaning up history
 -------------------
@@ -86,7 +102,6 @@ this. In summary:
 * There are other tools that are useful if your branch is too big for one squash.
 
 
-
 Details about ``PYTHONPATH``
 ----------------------------