Add notes on working against a maintenance branch

docs/csla-github-flow.md

@@ -28,6 +28,14 @@ Here's a tl;dr summary if you just want to get working without all the backgroun
    1. `git checkout -b 123-feature-branch marimer/master`
    1. Your local workspace is now in a feature branch based on the latest code in MarimerLLC/csla master
 
+## Create a branch to do some work _against a maintenance branch_
+1. Make sure your local clone is updated with MarimerLLC/csla
+   1. `git fetch marimer`
+1. Create a feature branch (work area) based on the upstream master
+   1. `git checkout -b 123-feature-branch marimer/<maintenance-branch-name>`
+   1. Your local workspace is now in a feature branch based on the latest code in the maintenance branch
+1. **Important:** when you submit your PR (later in this doc) make sure the target of your PR is _<maintenance-branch-name>_, not master
+
 ## Do your work
 1. Edit code, and do other stuff
 1. Commit your changes to your local clone