Add missing git command to pull request guide (#510)

* Add missing git command to pull request guide

The pull request step-by-step guide provides instructions to create a
git branch using upstream/master as the start-point.  This commit adds
a command to the guide which safely makes sure that this start-point
exists in the local repository.  Fixes #509.

* Clarify the need to configure git remote

This added comment explains the need for an existing remote to create a branch from.  The prerequisite git workflow is already documented.

Co-Authored-By: Kyle Stanley <[email protected]>

* Move extra git step to its own bullet point

Author: zthompson47

pullrequest.rst

@@ -64,6 +64,10 @@ Step-by-step Guide
 You should have already :ref:`set up your system <setup>`,
 :ref:`got the source code <checkout>`, and :ref:`built Python <compiling>`.
 
+* Update data from your ``upstream`` repository::
+
+     git fetch upstream
+
 * Create a new branch in your local clone::
 
      git checkout -b <branch-name> upstream/master