Development: Clarify PR creation using gh tool.

The existing documentation is slightly misleading in that it makes it
seem like the default repo should be the fork, rather than upstream,
when this may not be the case. Add a note to clarify this.

Resolves: #189

Author: InterLinked1

docs/Development/Policies-and-Procedures/Code-Contribution/index.md

@@ -120,6 +120,8 @@ You should always use option 1 when possible.  Unlike Gerrit, GitHub was never d
 
 When you've finished your work and committed, you can create a new pull request by running `gh pr create --fill --base <base branch>`.  The `--fill` option sets the pull request description to the same as the commit message and the `--base` option indicates which asterisk branch the pull request is targeted for (usually master).  When prompted where the new branch should be pushed, choose your fork, NOT the upstream repo.
 
+If you've already pushed the new branch to your fork, the default repository needs to be set to the repository to which you want to contribute the pull request, i.e. upstream, NOT your fork. If it's not already set, you can run `gh repo set-default asterisk/asterisk` to do this. Otherwise, the PR will be created against your fork, rather than the upstream repo.
+
 #### Cherry Picking
 Unless there are special circumstances, all changes need to be cherry-picked to the currently-supported major version branches.  This is accomplished by adding a special comment to the PR indicating which branches the PR should be cherry-picked to: